ROLE OF THE SPINAL-CORD IN GENERATING THE 2-HZ 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 rhyth m is generated by a neural oscillator in the medulla. In urethan-anest hetized 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 outpu t of this medullary network [he 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, exc itatory amino acid antagonist) and bicuculline (GABA(A) antagonist) bi laterally into RVLM produced little effect on the 2- to 6-Hz rhythm. I ntrathecal 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 excitato ry input to the spinal cord is required for the generation of basal SN D, patterning of this input is not critical for production of the 2- t o 6-Hz SND rhythm that, in this species, may be essentially of spinal origin.