IDENTIFICATION OF CARBONIC-ANHYDRASE ACTIVITY IN BULLFROG OLFACTORY RECEPTOR NEURONS - HISTOCHEMICAL-LOCALIZATION AND ROLE IN CO2 CHEMORECEPTION

The objectives of this study were to determine: (1) the frequency and distribution of carbonic anhydrase (CA) activity in the bullfrog nasal cavities, and (2) whether inhibition of nasal CA affects the olfactor y receptor response to CO2 or other odorants. It was found, using Hans son's staining technique, that some olfactory receptor neurons exhibit ed CA activity and that these CA-positive receptors were distributed t hroughout the nasal cavity with peak densities in the dorsal and ventr al sensory epithelial regions. To test for the role of CA in olfactory transduction, electro-olfactograms (EOGs) were recorded from the surf ace of the ventral sensory epithelium in response to 2-s pulses of 5% CO2 and amyl acetate before and after topical CA inhibition with aceta zolamide (10(-3) mol.l(-1)). In 52 bullfrogs, 1222 sites on the ventra l epithelium were tested resulting in 23 locations that exhibited a re sponse to 5% CO2. Inhibition of CA caused an immediate 65% reduction i n the EOG response to CO2 while the response to amyl acetate was not a ffected. These results, along with the histochemical localization of C A in some olfactory receptor neurons, indicate that CA plays a role in the detection of CO2 in frog olfactory neurons and that only a small population of olfactory receptor neurons are CO2 sensitive.