Neuronal mechanisms mediating the integration of respiratory responses to hypoxia.

The activation of peripheral chemoreceptors by hypoxia or electrical stimulation of the carotid sinus nerve elicited a hypoxic respiratory response consisting of both stimulatory and subsequent or simultaneous inhibitory components (hypoxic respiratory stimulation and depression). Both components have different time domains of responses (time-dependent response), providing an integrated respiratory response to hypoxia. This review has focused on the neuroanatomical and neurophysiological correlations responsible for these responses and their neuropharmacological mechanisms. Hypoxic respiratory depression is characterized by the initial activation of respiration followed by a progressive and gradual decline in ventilation during prolonged and/or severe hypoxic exposure (biphasic response). The responsible mechanisms for the depression are located within the central nervous system and may be dependent upon activity from peripheral chemoreceptor. Two underlying mechanisms contributing to the depression have been advocated. (1) Change in synaptic transmission: Within the neuronal network controlling the hypoxic respiratory response, hypoxia might induce the enhancement of inhibitory neurotransmission (modulation), disfacilitation of excitatory neruotransmission or both. (2) Change in the membrane property of respiratory neurons: Hypoxia might suppress the membrane excitability of respiratory neurons composing the hypoxic respiratory response via modulating ion channels, leading to hyperpolarization or depolarization blocking of the neurons. However, the quantitative aspects of Pao(2) (degree and duration of hypoxic exposure) to induce these changes and the susceptibility of both mechanisms to the Pao(2) level have not yet been clearly elucidated.