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 investiga ted in vivo and in an in vitro slice preparation generating respirator y 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 spontaneousl y breathing anaesthetized mice of different ages (postnatal (P) days 0 to >56). Glycine receptor antagonization with concentrations of stryc hnine 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 st rychnine (0.2-2 mu M) abolished regular rhythmic discharge in the phre nic nerve. Bicuculline (0.5-50 mu M) produced dose-dependent increases in inspiratory time, amplitude and cycle length of phrenic nerve disc harge in anaesthetized neonatal mice whereas both cycle length and dur ation of inspiratory activity were reduced in mature animals. In addit ion, in both neonates and mature mice low concentrations of bicucullin e (0.5-5 mu M) abolished phrenic nerve discharge intermittently. 3. Th e response of respiratory-modulated hypoglossal (XII) neurones recorde d in tilted sagittal slices from newborn and mature mice during blocka de of glycine and GABA(A) receptors was similar to the phrenic nerve c hanges observed in vivo: in slices from neonates, the rhythmic activit y 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 rhythmi c activity in preparations from mature mice. Bicuculline (1-50 mu M) p roduced a dose-dependent prolongation of burst duration and a slowing of rhythmic discharge in slices from neonatal mice whereas in mature m ice rhythmic XII bursts were shortened and their frequency increased. At all maturational stages, bicuculline (1-50 mu M) induced severe dis ruption of the regular rhythm of XII neurone activity causing maintain ed 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 bicucu lline or strychnine (1-5 mu M) indicating an absence of a maturational change in the sensitivity of GABA(A) and glycine receptors to their r espective antagonists. 5. We conclude that over the first 15 days of l ife in the mouse there is a dramatic increase in the relevance of glyc ine receptors for respiratory rhythm generation, and change in the fun ctional role of GABA(A) receptors within the respiratory network, whic h may provide a stabilizing influence on neurones within the respirato ry network from birth onwards.