Fig. 4.

Trigeminal chemoreceptors express T2R bitter-receptor mRNA and respond to stimulation with bitter-tasting ligands applied to the nasal passages. (a) Coexpression of mT2R8 (red, in situ hybridization) and gustducin (green, immunocytochemistry) in nasal SCCs. Although the gustducin immunoreactivity is evident throughout the cell, the in situ hybridization signal, which reveals the location of T2R8 mRNA, is perinuclear, where rough ER is typically located. (Scale bar, 10 μm.) (b) Relative chemosensory component (mean response to stimulus - mean response to saline) of the integrated neural activity in the ethmoid branch of the trigeminal nerve in anesthetized rats. The graph shows the percentage increase in neural activity during chemical stimulation compared with stimulation of the system with saline only. Responses to all compounds were significantly greater than responses to control (saline) solutions (^*, paired t test, P < 0.05; six animals). Denat, denatonium (0.01 M); quinine (0.01 M); and Cyclo., cycloheximide (0.01 M). Error bars indicate SEM. (c) Mean percentage depression in respiratory rate when saline or bitter-tasting substances are applied to the nasal epithelium. The rate decreases significantly (^*, paired t test, P < 0.05; nine animals) when the nasal epithelium is bathed in 0.01 M denatonium, quinine, or cycloheximide. Error bars indicate SEM. (d) Respiratory record in an anaesthetized rat is shown. The respiratory cycle is indicated by the sinusoidal purple line showing a basal respiration rate of approximately six breaths in 5 sec. A profound respiratory depression is evoked by 0.01 M cycloheximide. In this example, on application of cycloheximide, the steady prestimulus rate is drastically reduced to a near apnea (rate is less than one breath per 10 sec.). Shallow, but relatively normally paced respiration returns ≈10 sec after washout of the strong trigeminal stimulant.