ACTIVATION OF NEURONS IN RAT TRIGEMINAL SUBNUCLEUS CAUDALIS BY DIFFERENT IRRITANT CHEMICALS APPLIED TO ORAL OR OCULAR MUCOSA

To investigate the role of trigeminal subnucleus caudalis in neural me chanisms of irritation, we recorded single-unit responses to applicati on of a variety of irritant chemicals to the tongue or ocular mucosa i n thiopental-anesthetized rats. Recordings were made from wide dynamic range (WDR) and nociceptive-specific units in superficial layers of t he dorsomedial caudalis (0-3 mm caudal to obex) responsive to mechanic al stimulation and noxious heating of the ipsilateral tongue(''tongue' ' units) and from WDR units in ventrolateral caudalis (0-2 caudal to o bex) responsive to mechanical and noxious thermal stimulation of corne a-conjunctiva and frequently also surrounding skin (''cornea-conjuncti val'' units). The following chemicals were delivered topically (0.1 mi ) onto the dorsal anterior tongue or instilled into the ipsilateral ey e: capsaicin (0.001-1% = 3.3 x 10(-2) to 3.3 x 10(-5) Id), ethanol (15 -80%), histamine (0.01-10% = 9 x 10(-1) to 9 x 10(-4) M), mustard oil (allyl-isothiocyanate, 4-100% = 4 x 10(-1) to 10 M), NaCl(0.5-5 M), ni cotine (0.01-10% = 6 x 10(-1) to 6 x 10(-4) M), acidified phosphate bu ffer (pH 1-6), piperine (0.01-1% = 3.5 x 10(-2) to 3.5 x 10(-4) M), se rotonin (5-HT; 0.3-3% = 1.4 x 10(-1) to 1.4 x 10(-2) M), and carbonate d water. The dose-response relationship and possible tachyphylaxis wer e tested for each chemical. Of 32 tongue units, 31 responded to one or more, and frequently all, chemicals tested. The population responded to 75.3% of the various chemicals tested (less than or equal to 10 per unit). The incidence of responses was independent of the order of che micals tested, except for capsaicin, which reduced subsequent response s. Responses to histamine, nicotine, 5-HT, and ethanol had a more rapi d onset and shorter duration compared with capsaicin, acid, and mustar d oil. Responses to all chemicals increased in a dose-related manner. Successive responses to repeated application decreased significantly f or nicotine, 5-HT, capsaicin, and piperine. Spontaneous firing increas ed significantly 5-10 min after initial application of capsaicin. Of 3 1 corneal-conjunctival units, 29 responded to one or more chemicals, a nd the population responded to 65% of all chemicals tested. Responses increased in a dose-related manner for all chemicals, and successive r esponses decreased significantly for histamine, nicotine, ethanol, aci d, and capsaicin. Responses of tongue units to histamine and nicotine were reduced significantly by ceterizine (H1 antagonist) and mecamylam ine, respectively. Mecamylamine also significantly reduced responses o f corneal-conjunctival units to nicotine. Different classes of irritan t chemicals contacting the oral or ocular mucosa can activate individu al sensory neurons in caudalis, presumably via independent peripheral transduction mechanisms. Multireceptive units with input from the tong ue or cornea-conjunctiva exhibited a similar spectrum of excitability to different irritant chemicals. Such neurons would not be capable of discriminating among different chemically evoked irritant sensations b ut could contribute to a common chemical sense.