Hypoxic chemosensitivity of the cardiorespiratory regions of the rostral ventrolateral medulla (RVLM)

Recently, unique cardiorespiratory regions in the RVLM have been found to be oxygen sensitive [1,2]. However, the mechanism of sensing O₂ in these RVLM regions is unknown. Since heme oxygenase (HO) has been shown to be involved in the hypoxic responses of the carotid body and pulmonary artery, the aim of our work has been to determine if HO is present in the RVLM, whether expression of HO is altered by chronic hypoxia, and whether HO is necessary for the excitatory response of RVLM neurons. RT-PCR of the RVLM of rats exposed to hypoxia (10% O₂) or normoxia for 10 days, revealed that HO-2 is expressed in the RVLM constitutively during both normoxia and hypoxia and that HO-1 is induced during chronic hypoxia [3]. Immunocytochemistry localized these HO isoforms to the C1 region and pre-Bötzinger complex (pre-Böt) of the RVLM. Thus, HO isoforms are present in the RVLM cardiorespiratory regions under control and hypoxic conditions, consistent with a potential role for HO in the O₂ sensing function of these cardiorespiratory regions. To determine whether HO is important for the oxygen sensitivity of the RVLM, we examined the response of cultured RVLM neurons to chemical hypoxia (NaCN), before and after blocking HO with SnPP-IX, and correlated the hypoxia-excited response with the expression of HO-2 using immunocytochemistry. Primary cultures were prepared from neonatal rats and studied using the whole-cell perforated patch clamp technique. The hypoxia-induced depolarization of these RVLM neurons in response to NaCN was abolished after blocking HO. Examination of these hypoxia-excited neurons showed that they were immunoreactive for HO-2. Further phenotyping found that RVLM neurons that express HO are located in the tyrosine hydroxylase rich C1 sympathoexcitatory region as well as within the neurokinin-1 receptor (NK-1R) rich pre-Böt. In fact, 70% of the neurons expressing HO-2 in the pre-Böt co-express NK-1R suggesting that oxygen sensitivity may reside in some respiratory rhythm generating neurons in the pre-Bötzinger complex. Thus, HO is present in the cardiorespiratory regions of the RVLM, HO-1 is induced by chronic hypoxia, and HO is necessary for the excitatory response to chemical hypoxia in RVLM neurons consistent with the presence of HO-2 in these excited neurons. These findings support an important role for HO in the oxygen sensitivity of cardiorespiratory neurons in the RVLM.