Synaptic inhibition in the isolated respiratory network of neonatal rats

Gramicidin-perforated patch-clamp recording revealed phasic Cl--mediated hy perpolarizations in respiratory neurons of the brainstem-spinal cord prepar ation from newborn rats. The in vitro respiratory rhythm persisted after bl ock of gamma-aminobutyric acid (GABA), i.e. GABA(A), receptor-mediated inhi bitory postsynaptic potentials (IPSPs) with bicuculline and/or glycinergic IPSPs with strychnine. In one class of expiratory neurons, bicuculline unma sked inspiration-related excitatory postsynaptic potentials (EPSPs), leadin g to spike discharge. Bicuculline also blocked hyperpolarizations and respi ratory arrest due to bath-applied muscimol, whereas strychnine antagonized similar responses to glycine. The reversal potential of respiration-related IPSPs and responses to GABA, muscimol or glycine was not affected by CO2/H CO3--free solutions, but shifted from about -65 mV to values more positive than -20 mV upon dialysis of the cells with 144 instead of 4 mM Cl-. Impair ment of GABA uptake with nipecotic acid or glycine uptake with sarcosine ev oked a bicuculline- or strychnine-sensitive decrease of respiratory frequen cy which could lead to respiratory arrest. Also, the GABA(B) receptor agoni st baclofen led to reversible suppression of respiratory rhythm. This in vi tro apnoea was accompanied by a K+ channel-mediated hyperpolarization (reve rsal potential -88 mV) of tonic cells, whereas membrane potential of neighb ouring respiratory neurons remained almost unaffected. Both baclofen-induce d hyperpolarization and respiratory depression were antagonised by 2-OH-sac lofen, which did not affect respiration-related IPSPs per se. The results s how that synaptic inhibition is not essential for rhythmogenesis in the iso lated neonatal respiratory network, although (endogenous) GABA and glycine have a strong modulatory action. Hyperpolarizing IPSPs mediated by GABA(A) and glycine receptors provide a characteristic pattern of membrane potentia l oscillations in respiratory neurons, whereas GABA(B) receptors rather app ear to be a feature of non-respiratory neurons, possibly providing excitato ry drive to the network.