Role of rostral medulla in serotonin-induced changes of respiratory rhythm in newborn rat brainstem-spinal cord preparations

- We recorded respiratory-like nerve activity in Pons-medulla-spinal cord in vitro.
- Rostral ventrolateral medulla (RVLM) is sensitive to 5-HT application.
- Application of 10 μM 5-HT increased the activity before but not after transection.
- 5-HT suppressed the rhythm after transection at the level of the facial nucleus.
- Without the pons, 5-HT suppresses respiratory rhythm through a distinct RVLM region.

Abnormalities of the serotonin (5-hydroxytryptamine, 5-HT) system may induce respiratory disorders. We examined which regions in the rostral medulla are important for the effect of 5-HT on the frequency of respiratory-like nerve (fR-like) activity by transecting the preparations at different levels near the facial nucleus (nVII) in newborn rat brainstem-spinal cord preparations. The fR-like activity at the fourth cervical ventral root (C4) of the Pons-medulla-spinal cord preparations in 2-3-day-old rats (n = 25) was monitored at 26 °C, and the change in fR-like activity in response to application of 10 μM 5-HT before and after transection was compared among three groups, in which nVII was retained (group A, n = 10), partially retained (group B, n = 7), or eliminated (group C, n = 8) by the transection. Before transection, the resting fR-like activity (set to 100%) and stimulant effect of 5-HT (+101-143%) were similar among the groups. After transection, resting fR-like activity increased in all groups, but the facilitatory effects of 5-HT on the fR-like activity were abolished in groups A and C (fR-like activity of −4% and +7%, respectively). In group B, 5-HT became inhibitory (fR-like activity of −28%). In conclusion, a distinct part of the rostral medulla in the absence of pontine influences may mediate the inhibitory effects of 5-HT on the respiratory rhythm.