Controlled noxious chemical stimulation: responses of rat trigeminal brainstem neurones to CO2 pulses applied to the nasal mucosa. 
The nasal mucosa of halothane-anesthetized rats was stimulated with defined CO2 pulses.  Recordings were performed from single trigeminal brainstem neurons to characterize their responses to this controlled chemical irritation.  All cells examined with this stimulus displayed graded discharges to increasing concentrations of CO2.  Enhanced responses were obtained in a group of neurons when the duration of the interstimulus interval was increased.  The application of chemical irritants, notably mustard oil or acetic acid induced vigorous ongoing discharges in all cells tested.  The CO2 stimulation method described here thus provides an ideal model for the quantitative physiological and pharmacological examination of chemically induced nociception.