Odorants presented to the rat nasal cavity increase cortical blood flow

Complaints about unpleasant environmental odorants, both outdoor and indoor, are increasingly being reported. The main complaints of health symptoms from environmental odorants are eye, nose and throat irritation, headache and drowsiness. Complaints may arise from the stimulation of olfactory receptors or trigeminal chemoreceptors. Stimulation of cerebrovascular nociceptors originating from a branch of the trigeminal nerve may be associated with an increase in cortical blood flow which is thought to be related to headache. Since odorants are reported to elicit headaches, the possibility that odorants may increase cortical blood flow was examined. Cortical blood flow was monitored in rats using a laser-Doppler flowmeter. The flowmeter probe was placed over the left frontal cortex while propionic acid, cyclohexanone, amyl acetate or butanol was delivered to the nasal cavity via an olfactometer. Cortical blood flow increased as the concentration increased for three of the odorants tested. The greatest increase in blood flow occurred to the presentation of propionic acid, followed by cyclohexanone and amyl acetate. There was no response to butanol. These data demonstrate that odorants can alter cerebrovascular blood flow, which may account, in part, for one of the health symptoms reported for odorants.