Modulation of sensory irritation responsiveness by adenosine and malodorants

Respiratory tract reflex responses are an important defense mechanism against noxious airborne materials. This study was aimed at defining the effects of adenosine on sensory irritation responsiveness and its role in odorant-irritant interactions. These experiments were aimed at testing the hypothesis that adenosine, through the A2 receptor, enhances trigeminal nerve responses to multiple irritants and that odorants enhance responsiveness to irritants through A2 pathways in the female C57Bl/6 mouse. The adenosine precursor, AMP, immediately and markedly increased the sensory irritation response to capsaicin, cyclohexanone, and styrene, irritants that activate chemosensory nerves through differing receptor pathways. The neuromodulatory effect was blocked by the general adenosine receptor antagonist theophylline and by the A2 receptor-specific antagonist DMPX. Multiple odorants were examined, including R-carvone (spearmint), linalool (lavender), trimethylamine (rotting fish), mercaptoethanol, and ethyl sulfide (stench and rotten eggs). Of these, only mercaptoethanol and ethyl sulfide exhibited neuromodulatory effects, enhancing the sensory irritation response to styrene or cyclohexanone. This effect was blocked by theophylline and DMPX indicating the importance of adenosine A2 receptor pathways in this effect. These results highlight that trigeminal chemosensory responsiveness is not static, but can be quickly modulated by adenosine and select odors resulting in hyperresponsive states.