TONIC SYMPATHETIC CHEMOREFLEX AFTER BLOCKADE OF RESPIRATORY RHYTHMOGENESIS IN THE RAT

1. We sought to determine whether the increase in sympathetic nerve di scharge (SND) caused by carotid chemoreceptor stimulation requires the integrity of ventrolateral medullary structures involved in generatin g respiratory rhythm and pattern. Experiments were done in urethane-an aesthetized, vagotomized, aortic deafferented, ventilated rats except when indicated (see paragraph 3). 2. Brief hypoxia (N-2 for 5-12 s) or I.V. NaCN (50-100 mu g kg(-1)) activated SND in bursts synchronized w ith the phrenic nerve discharge (PND). No effect was produced in chemo deafferented rats. 3. In unanaesthetized vagotomized decerebrated rats , ligation of the internal carotid arteries preserved peripheral chemo receptor function but abolished baroreflexes. In this preparation, sti mulation of peripheral chemoreceptors (N-2 for 2-6 s) also activated S ND in bursts synchronized with PND. 4. Bilateral microinjection of the GABA(A) receptor agonist muscimol into the caudal ventrolateral medul la (CVLM) instantly blocked the sympathetic baroreflex, eliminated PND at rest and during chemoreceptor stimulation but did not change the m ean increase in SND produced by chemoreceptor stimulation. Sympathoact ivation in response to chemoreceptor stimulation became tonic after 1- 13 min and was still totally dependent on the integrity of the carotid sinus nerves.5. Muscimol injection instantly eliminated the respirato ry outflow of the Xth and XIIth cranial nerves, both at rest and durin g chemoreceptor stimulation. 6. Muscimol eliminated the on-off respira tory pattern of neurons in the rostral ventrolateral medulla (RVLM). D uring chemoreceptor stimulation, these cells became activated or inhib ited tonically. 7. Muscimol injection raised the resting discharge rat e of vasomotor presympathetic cells in RVLM, blocked their barorecepto r inputs but did not change the magnitude of their excitation by chemo receptor stimulation. Muscimol injection eliminated their respiratory modulation. 8. In conclusion, the sympathetic response to chemorecepto r stimulation may be due to convergence and integration in RVLM of two processes: respiration-independent excitatory input to RVLM neurons a nd respiratory patterning of their activities via inputs from the pre- Botzinger complex.