Retrotrapezoid nucleus glutamate injections: long-term stimulation of phrenic activity

In chloralose-urethan anesthetized, paralyzed, vagotomized, glomectomized, and servo-ventilated cats we examined the effects of 10 nl of glutamate (10 mM, 100 mM, and 1 M) injected unilaterally over 60 s into the region of the retrotrapezoid nucleus (RTN). Seven 10 mM glutamate injections produced no consistent effects on the amplitude of the integrated phrenic nerve signal, respiratory cycle duration, or blood pressure. Ten 100 mM injections consistently increased integrated phrenic amplitude significantly from a baseline average of 31 +/- 2% (SE) of maximum to a peak response average of 50 +/- 3% of maximum. This effect was long lasting (45.6 +/- 8.6 min). Blood pressure responses were variable. Seven 1 M glutamate injections consistently decreased integrated phrenic amplitude significantly from a baseline average for all injections of 29 +/- 3% of maximum to a peak average of 20 +/- 5% of maximum. Respiratory cycle duration and blood pressure responses were variable. Prior injection into the RTN of 10 nl of 100 mM kynurenic acid attenuated the subsequent response of the integrated phrenic amplitude to injection of 10 nl of glutamate at the same site. Comparison of glutamate (10 nl, 100 mM) injected over 60 s vs. 30 ms showed that the prolonged increase in phrenic activity was related to the longer-duration (60-s) injections and that RTN single units were stimulated for up to 5 min on average after the 60-s injection with one unit showing an increase in firing rate over 40 min. After the 30-ms injection, integrated phrenic amplitude and RTN unit mean firing rate were increased for the first two breaths and at 5 min after the injection. We conclude that glutamate injected into the RTN increases local single-unit firing rate and the amplitude of the integrated phrenic activity. Injections made over 60 s result in prolonged phrenic stimulation and, in some cases, in RTN single-unit firing rate.