The postnatal period is characterized by relative respiratory instability. Three recent reports have shown that in the rat the ventilatory response to carbon dioxide (CO
2) declines at P6–P7 compared to earlier and later postnatal ages. Stunden et al [
1] found that the percent increase in minute volume (V
E) induced by 5% CO
2 declined from ~65% at P2 to ~10% at P8. Thereafter it gradually increased to ~70% at P18.5. These authors also determined the slope of minute ventilation at 1% CO
2 increases between 1 and 5% and found a fall from 240 ml/min/kg/% CO
2 at P1 to 27 ml/min/kg/% CO
2 at about P8, with a subsequent increase to 67 ml/min/kg/% CO
2 at P21. The increases in V
E were due to an increase in tidal volume with little change in respiratory frequency. Similarly Serra et al [
2] showed that 7% CO
2 resulted in a 25% increase in V
E in animals studied between P1 and P3. In these rats at P6–P7, however, hypercapnia failed to increase V
E. In the same animals V
E increased ~25% at P12–P13 and almost 100% at P18–P19. The components of minute ventilation, tidal volume and frequency were not seperately reported. Abu-Shaweesh and co-authors [
3] examined rats between P5 and P41–42. In 5% CO
2 V
E increased ~40% at P5 then rose to a plateau of ~100% at P22–23. In the younger rats only tidal volume increased while in those at P22 – P23 both frequency and tidal volume increased. These authors showed a second altered response to CO
2 in the younger animals. At P5 rats differed from those at other ages in that they were unable to sustain their respiratory rate increased during a 5 min exposure. These later authors found that the GABAA receptor blocker bicuculline prevented the lengthening of expiratory time (T
E) which characterized the decline in respiratory rate during hypercapnia. The actions of bicuculline are not confined to GABAA receptors as this agent also inhibits small-conductance calcium-activated potassium channels [
4]. In addition when administered systemically bicuculline induces an increase in respiratory drive under basal conditions. Thus inspiratory time (TI) and T
E are shortened [
3] and peak phrenic nerve activity is increased about 3 fold [
5]. Therefore it is not clear that the bicuculline effects are due to blocking the GABA inhibition which occurs in hypercapnia or to a generalized stimulation of respiratory activity.
GABA is synthesized by two GAD isoforms, GAD65 and GAD67. Their subcellular localization has suggested that each may have a distinct function. GAD65 is localized to axon terminals where it is bound to the synaptic vesicle membrane. GAD67 is distributed in the cytosol throughout the cell [
6,
7]. Electrophysiological studies (see Discussion for details) have shown that mice lacking GAD65 release GABA in a normal fashion under basal conditions. During sustained stimulation, however, the probability of release of the inhibitory neurotransmitter is significantly reduced [
8].
The present studies were undertaken to determine if: 1) mice show a similar decline in CO2 sensitivity in the first postnatal week as is seen in rats; and 2) mice lacking GAD65 are ale to sustain an increase in respiratory rate during an acute exposure to elevated CO2.