It is well established that GABAA-mediated postsynaptic potentials are excitatory in many brain regions during embryonic and early postnatal life. The pre-Bötzinger complex (PBC) in the brainstem is an essential component of the respiratory rhythm-generating network, where GABAA-mediated inhibition plays a critical role in generating a stable respiratory rhythm in adult animals. In the present study, using the perforated patch technique, we investigated the maturation of GABAA receptor-mediated effects on rhythmically active PBC neurons and on the motor output in slice preparations from P0-15 neonatal mice. The reversal potential of GABAA receptor-mediated current (EGABA-A) switched from depolarizing to hyperpolarizing within the first postnatal week. EGABA-A was -13.7 +/- 9.8 mV at P0, then it changed to -44.8 +/- 7.0 mV at P2 and -71.5 +/- 6.8 mV at P4. Perfusion of bicarbonate-free saline has no detectable influence on EGABA-A, indicating that a lack of Cl- extrusion during P0-3 is mainly responsible for early GABAA-ergic excitation. At the network level, blockade of GABAA receptors with bicuculline did not significantly change the frequency of rhythmic bursts recorded from hypoglossal nerve roots before P3, whereas it increased the coefficient of variation. After P3, bicuculline increased burst frequency with little effect on the coefficient of variation. Thus, chloride-mediated inhibition, which appears in PBC neurons after P3, coincides with the appearance of GABAA-mediated modulation of the respiratory rhythm. GABAA receptor-activated inhibition may therefore be necessary for frequency modulation in the respiratory network beginning on the fourth postnatal day in the mouse brainstem.

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABAA‐receptor‐ and glycine‐receptor‐mediated inhibition is not present, thus the question arises as to whether GABAB‐receptor‐mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0‐P15), we analysed the role of GABAB‐mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25–2 mm) reduced the respiratory frequency. This reduction was prevented by the selective GABAB receptor antagonist CGP55845A (CGP) alone at P0‐P3, but by bicuculline as well as CGP at P7‐P15. Blockade of GABAB receptors by CGP increased the respiratory frequency at P0‐P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABAB‐receptor‐mediated pre‐ and postsynaptic modulation was examined by analysing the effect of GABAB receptors on spontaneous and miniature IPSCs. In younger animals (P0‐P3), the GABAB receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7‐P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABAB‐receptor‐mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABAB‐receptor‐mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.

GABAB receptors modulate respiratory rhythm generation in adult mammals. However, little is currently known of their functional significance during postnatal development. In the present investigation, the effects of GABAB receptor activation on voltage-activated Ca2+ currents were examined in rhythmically active neurons of the pre-Bötzinger complex (PBC). Both low- (LVA) and high-voltage-activated (HVA) Ca2+ currents were present from the first postnatal day (P1). The density of LVA Ca2+ currents increased during the first week, whilst the density of HVA Ca2+ currents increased after the first week. In the second postnatal week, the HVA Ca2+ currents were composed of L- (47 +/- 10%) and N-type (21 +/- 8%) currents plus a 'residual' current, whilst there were no N-type currents detectable in the first few days. The GABAB receptor agonist baclofen (30 microM) increased LVA Ca2+ currents (30 +/- 11%) at P1-P3, but it decreased the currents (35 +/- 11%) at P7-P15 without changing its time course. At all ages, baclofen (30 microM) decreased the HVA Ca2+ currents by approximately 54%. Threshold of baclofen effects on both LVA and HVA Ca2+ currents was 5 microM at P1-P3 and lower than 1 microM at P7-P15. The effect of baclofen was abolished in the presence of the GABAB receptor antagonist CGP 55845A (50 nM). We conclude that both LVA and HVA Ca2+ currents increased postnatally. The GABAB receptor-mediated modulation of these currents undergo marked developmental changes during the first two postnatal weeks, which may contribute essentially to modulation of respiratory rhythm generation.

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