The hypoxic response of neurones within the in vitro mammalian respiratory network

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Abstract
The transverse brainstem slice preparation containing the pre‐Bötzinger complex (PBC) was used in mice to study developmental changes of the response of the in vitro respiratory network to hypoxia. This preparation generates at different postnatal stages (postnatal days (P) 0‐22) spontaneous rhythmic activity in hypoglossal (XII) rootlets that occur in synchrony with periodic bursts of neurones in the PBC. In slices from P0‐4 mice, hypoxia did not significantly affect the amplitude of rhythmic synaptic drive potentials in four of five inspiratory neurones. Hypoxia reduced, but did not suppress, the amplitude of synaptic drive potentials in only one inspiratory neurone. Spike discharge and phasic ‘inspiratory’ hyperpolarizations of six expiratory neurones were suppressed during hypoxia revealing a phasic ‘inspiratory’ depolarization. The coupling between rhythmic activity in PBC neurones and XII bursts occurred under control conditions in preparations from P0‐4 mice in a 1:1 manner (n= 11) and from mice older than P5 in a 3:1 manner (n= 9). During hypoxia, PBC and XII activity were linked in a 1:1 manner in all slices. In six of fourteen inspiratory PBC neurones, the amplitude of synaptic drive potentials of slices from mice older than P8 was increased during the period of augmentation, reduced during the period of depression and suppressed during a hypoxic response which we refer to as central apnoea. Augmentation led to a weak‐to‐moderate membrane depolarization which on average was 4.8 ± 3.7 mV. This depolarization was followed by a hyperpolarization of 6.2 ± 4.1 mV only in four inspiratory neurones. In the majority of neurones (n= 9), however, membrane depolarization remained stable and was not followed by hyperpolarization. In expiratory neurones (n= 12) from this age group hypoxia suppressed phasic hyperpolarizations that occurred in synchrony with XII bursts. As similarly seen in inspiratory neurones, membrane potentials were depolarized by 5.1 ± 4.1 mV during the period of hypoxic augmentation. The hypoxic response of respiratory neurones within the pre‐Bötzinger complex resembles the response of neurones that were previously described under in vivo conditions. Thus we conclude that the ‘transverse rhythmic slice’ is a good model for studying the hypoxic response of the respiratory network under in vitro conditions.