Activation of nitric oxide-producing neurons in the brain stem during cardiac sympathoexcitatory reflexes in the cat

Abstract

Our previous studies have shown that selective inhibition of nitric oxide in the brain reduces pressor responses to activation of cardiac sympathetic afferents, thus suggesting that nitric oxide is involved in central regulation of cardiac-cardiovascular sympathoexcitatory reflexes. Central neural regions in which nitric oxide-producing neurons are activated during these reflexes have not been well characterized. In the present study, we located nitric oxide-producing neurons in the brain stem activated by the input from cardiac sympathetic afferents by detecting colocalization of c-Fos immunoreactivity with nitric oxide synthesizing neurons. Expression of c-Fos has been used as a marker of activated neurons. Nitric oxide-producing neurons were identified by histochemical labeling of nicotine adenine dinucleotide phosphate-diaphorase (NADPH-d). In anesthetized cats with bilateral barodenervation and cervical vagotomy, bradykinin (1–10 μg in 0.1 ml; n=6) was applied to the anterior surface of the left ventricle six times every 20 minutes. Repetitive application of bradykinin consistently increased blood pressure, while the vehicle for bradykinin (0.9% saline, n=5) produced no responses. A substantial fraction (6–27%) of NADPH-d positive neurons displayed Fos immunoreactivity in the nucleus of the solitary tract, caudal and rostral ventral lateral medulla, lateral tegmental field, locus coeruleus and parabrachial nucleus in the bradykinin-treated cats. However, either no or rare (1–4%) double-labeled cells were found in these regions in control animals. Thus, nitric oxide-producing neurons are activated in several regions in the brain stem during stimulation of cardiac sympathetic afferents by bradykinin. Our data suggest that nitric oxide functions as a neurotransmitter/modulator in these areas to regulate the cardiac sympathoexcitatory reflexes.