Basis for the naturally occurring activity of rostral ventrolateral medullary sympathoexcitatory neurons

This chapter entertains two theories (network oscillator vs. intrinsic pacemaker) on the generation of the major component of SND - the 2- to 6-Hz rhythm in the cat and the 2- to 10-Hz rhythm in the rat. Evidence from experiments on cats strongly supports the hypothesis that the 2- to 6-Hz rhythm is an emergent property of a network oscillator located in the brain stem. Specifically, we propose that the 2- to 6-Hz component in SND arises from an ensemble of neurons of different types interconnected in such a way to generate the rhythm. There is no evidence for the existence of intrinsic pacemaker neurons in this network. Our experiments on cats also indicate that the rhythm is generated by neurons in the brain stem that are antecedent to RVLM-spinal SE and raphespinal-SI neurons. Our experiments on rats are not consistent with the hypothesis of Sun et al. (1988b) that "vasoconstrictor and cardioaccelerator sympathetic tone is due in large part to the intrinsic pacemaker activity of a small group of reticulospinal excitatory neurons located at the extreme anterior tip of the ventrolateral medulla". Rat RVLM pacemaker neurons act independently of each other following i.c. injection of the glutamate-receptor antagonist, KYN (Sun et al., 1988a). Thus, they would as a group provide white noise input to spinal neurons. It follows that i.c. KYN should have desynchronized SND if, as proposed by Sun et al., RVLM pacemakers are primarily responsible for SND in the rat. This was found not to be the case. The most likely explanation is that the 2- to 10-Hz rhythm in rat SND is not generated by intrinsic pacemaker neurons in the RVLM.