ROLE OF FAST INHIBITORY SYNAPTIC MECHANISMS IN RESPIRATORY RHYTHM GENERATION IN THE MATURING MOUSE

1. The importance of glycinergic and GABA(A)ergic synaptic mechanisms for respiratory rhythm generation in the maturing mouse were investigated in vivo and in an in vitro slice preparation generating respiratory rhythmic activity spontaneously at all postnatal ages. 2. The effect on respiration of topical application of strychnine or bicuculline to the surface of the ventrolateral medulla was assessed in spontaneously breathing anaesthetized mice of different ages (postnatal (P) days 0 to >56). Glycine receptor antagonization with concentrations of strychnine up to 25 mu M was ineffective in altering the breathing pattern in neonates (P1-P8). However, in mature mice (P > 15), low doses of strychnine (0.2-2 mu M) abolished regular rhythmic discharge in the phrenic nerve. Bicuculline (0.5-50 mu M) produced dose-dependent increases in inspiratory time, amplitude and cycle length of phrenic nerve discharge in anaesthetized neonatal mice whereas both cycle length and duration of inspiratory activity were reduced in mature animals. In addition, in both neonates and mature mice low concentrations of bicuculline (0.5-5 mu M) abolished phrenic nerve discharge intermittently. 3. The response of respiratory-modulated hypoglossal (XII) neurones recorded in tilted sagittal slices from newborn and mature mice during blockade of glycine and GABA(A) receptors was similar to the phrenic nerve changes observed in vivo: in slices from neonates, the rhythmic activity of XII neurones was resistant to concentrations of strychnine up to 50 mu M whereas low doses of strychnine (0.2-2 mu M) abolished rhythmic activity in preparations from mature mice. Bicuculline (1-50 mu M) produced a dose-dependent prolongation of burst duration and a slowing of rhythmic discharge in slices from neonatal mice whereas in mature mice rhythmic XII bursts were shortened and their frequency increased. At all maturational stages, bicuculline (1-50 mu M) induced severe disruption of the regular rhythm of XII neurone activity causing maintained depolarizations and oscillations in membrane potential. 4. On-going inhibitory postsynaptic potentials of neurones located in the ventral respiratory group region of tilted sagittal slices from both immature and mature mice were sensitive to low concentrations of either bicuculline or strychnine (1-5 mu M) indicating an absence of a maturational change in the sensitivity of GABA(A) and glycine receptors to their respective antagonists. 5. We conclude that over the first 15 days of life in the mouse there is a dramatic increase in the relevance of glycine receptors for respiratory rhythm generation, and change in the functional role of GABA(A) receptors within the respiratory network, which may provide a stabilizing influence on neurones within the respiratory network from birth onwards.