Role of ventrolateral medulla in generating the 10-Hz rhythm in sympathetic nerve discharge

Susan M. Barman and Gerard L. Gebber
Am J Physiol Regul Integr Comp Physiol 293: R223–R233, 2007.

The main focus of this study is how inputs of selective and non-selective excitatory amino acid receptor antagonists affect the ventrolateral medulla (VLM) in generating a 10-Hz rhythm in sympathetic nerve discharge. Using urethane-anesthetized, baroreceptor-denervated cats, they would record the changes of frequency in both the right inferior cardiac and  left inferior cardiac nerves. After unilateral injections of bicuculline (GABA-A receptor antagonists) were giving 1 and 5 mm rostral to the obex in the VLM, they found it would completely remove or reduce the 10-Hz rhythm on both sides of the body. Similarly they did unilateral injections kynurenate, (nonselective excitatory amino acid receptor antagonist) but this time did the injections both caudal or rostral in the VLM. Results found that the 10-Hz frequency was reduced but not eliminated. This study shows that GABAerginc transmission is in some way necessary for this 10-Hz rhythm to occur in sympathetic nerve discharge and that both rostral and caudal GABAerginc neurons play a role. - ZAK

Renal And Cardiac Sympathetic Baroreflexes In Hypertensive Rabbits

Geoffrey A Head and Sandra L Burke
Clin Exp Pharmacol Physiol. 2001 Dec;28(12):972-5.

  In this study, rabbits either had a clip fixed around one renal artery to induce hypertension (known as the two kidney one clip or 2k1c technique) or were given a sham surgery. The rabbits’ renal sympathetic nerve activity (RNSA) and heart rate (HR) were compared between groups during infusion of drugs to raise or lower blood pressure (phenylephrine and nitroprusside, respectively).

  At 3 weeks after surgery, there was a strong reduction in the range/maximum nerve activity during decreases in blood pressure (36% lower than sham controls). However the maximum RSNA returned to levels near sham controls by 6 weeks. The resting point and curves for RSNA and HR were both shifted to the right, suggesting an attenuation in sensitivity in both parameters, at 3 and 6 weeks. This brings up questions in my mind about what structures of the baroreflex are being attenuated - because from a baroreceptor-centric point of view, the interpretation is correct... but from a very RVLM-centric point of view, you might try to argue that the baroreflex is actually being facilitated by 2k1c.

  Most interesting is that they say their results conflict with another group's paper showing an INCREASE in maximum RSNA after 2k1c. This was a difference in how the change was quantified. In the other paper, the change was measured as a percent of resting nerve activity, while in this paper it was measured as a percent of the max RSNA activated by a noxious stimulus (smoke). When the authors of this paper adapted the other way of measuring RSNA, they got the same results as the other group. So, how you normalize can make a lot of difference in interpreting your results. - DJH