Type B olfactory sensory neurons of the main olfactory epithelium have a distinctive expression profile, most notably expressing GUCY1B2, CNGA2, and TRPC2 (a cation channel typically expressed by vomeronasal sensory neurons), but not expressing ADCY3 or odorant receptors, which usually are expressed by olfactory sensory neurons. Type B cells have been shown to respond rapidly and in a graded manner to reduced oxygen levels and this response requires functional GUCY1B2 and TRPC2, as well as extracellular calcium cations. Type B cells from wild type mice flux calcium in response to 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, and 8-bromo-cGMP, but were found not to respond to forskolin, carbon disulfide, mouse urine, or an array of other odorants. Inhibition of nitric oxide synthases did not interfere with the response of type B cells to reduced oxygen. Strong expression of Gucy1b2 in the brain appears restricted to this subset of olfactory sensory neurons.
Type B cells from mice homozygous for this targeted null allele of Gucy1b2 respond normally to 8-bromo-cGMP or KCl, but fail to flux calcium in response to reduced oxygen. The impact of this was evidenced by assessing the activation of postsynaptic olfactory bulb neurons in the vicinity of type B cell glomeruli subsequent to exposure to environmental hypoxia. These post-synaptic olfactory bulb neurons showed increased FOS immunoreactivity, indicative of activation, in controls and heterozygotes but not in homozygotes. The essential role for GUCY1B2 in environmental hypoxia sensing was further confirmed in a conditioned place aversion test. Wild-type controls and heterozygotes responded with place aversion to a reduced oxygen area, where homozygotes did not. Homozygotes on the 129S6 congenic background were found to have a normal hyperventilation response to low environmental oxygen, consistent with normal carotid body function, further proving that Type B cells assess the oxygen content of the air, not the oxygenation in the body. Homozygotes performed normally in a Morris water maze for both latency to reach the platform and reference memory.
