Role of the spinal cord in generating the 2- to 6-Hz rhythm in rat sympathetic outflow.

In baroreceptor-denervated animals, sympathetic nerve discharge (SND) displays a 2- to 6-Hz rhythm. Current theories suggest that this rhythm is generated by a neural oscillator in the medulla. In urethan-anesthetized rats, we have examined the effect on the 2- to 6-Hz rhythm of lumbar SND produced by 1) altering the firing pattern of a major output of this medullary network [the rostral ventrolateral medulla (RVLM)] and 2) disrupting the interactions between medulla and spinal cord (SC). Microinjection of muscimol [gamma-aminobutyric acid (GABA) agonist] unilaterally or a mixture of kynurenic acid (KYN; broad spectrum, excitatory amino acid antagonist) and bicuculline (GABAA antagonist) bilaterally into RVLM produced little effect on the 2- to 6-Hz rhythm. Intrathecal injection of KYN or transection of the cervical SC also had little effect once SND had been restored by intrathecal injection of kainic acid (excitatory amino acid agonist). Thus, whereas an excitatory input to the spinal cord is required for the generation of basal SND, patterning of this input is not critical for production of the 2- to 6-Hz SND rhythm that, in this species, may be essentially of spinal origin.