Neuroscience
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Many chemicals including nicotine, capsaicin and piperine (pungent chemicals in red and black peppers, respectively) evoke oral pain and irritation via largely unknown neural mechanisms. As a first step in defining the central pathway for oral chemical irritation, we have used an immunohistochemical method to map locations of brainstem neurons expressing the nuclear protein, c-Fos (a putative nociceptive marker), following application of various irritants to the tongue. In barbiturate-anesthetized rats, one of the following was applied to the dorsal surface of the tongue: nicotine (0.5%), capsaicin (0.1%), histamine (2 or 20%), piperine (0.2%), acetylcholine (10%) or vehicle control (0.9% saline, dH2O, 70% ethanol). ⋯ Fos-immunoreactivity was also seen consistently in the ventrolateral medulla dorsal to the lateral reticular nucleus, and vestibular and cochlear nuclei, and less consistently in nucleus raphe pallidus and inferior olive, in both irritant and in control groups, indicating that it was not stimulus-evoked. These results have identified a population of neurons in the dorsomedial trigeminal nucleus caudalis likely to be involved in signaling chemical irritation of the tongue. Increases in Fos-immunoreactivity observed in the nucleus of the solitary tract, area postrema, and ventrolateral trigeminal caudalis also suggest roles for these areas in autonomic responses consequent to oral irritation.