Selective enhancement of glutamate-mediated pressor responses after GABAA receptor blockade in the RVLM of sedentary versus spontaneous wheel running rats

Patrick J. Mueller and Nicholas A. Mischel

Many previous studies have shown that sedentary conditions can result in enhanced nerve activity following activation of the rostral ventrolateral medulla (RVLM). This means that there is some type of mechanism that differs between active and sedentary animals in terms of the excitation and inhibition that occurs in the RVLM. The RVLM is primarily regulated by glutamate, an excitatory neurotransmitter, and GABA, an inhibitory neurotransmitter. This study was trying to determine how the glutamate could directly excite the RVLM, and the effects that blocking the GABA pathway may have on the response to glutamate. The hypothesis was that sedentary conditions would enhance sympthoexcitatory response to direct activation of the RVLM, with the responses increasing further by blocking tonic GABAergic transmission.

The methods of the experiment were similar to our microinjection protocols in terms of exposing the RVLM and recording nerve activity and blood pressure. The first protocol involved injection 1, 10, and 100mM concentrations of glutamate into the RVLM while recording the blood pressure, heart rate, and lumbar sympathetic nerve activity (LSNA). The second protocol administered a GABA receptor blocker called bicuculline before glutamate injections were performed. The glutamate was injected 5, 10, 15, 30, and 45 minutes after the bicuculline was administered to investigate the changes in nerve activity after certain periods of time. The third protocol was a control section in which glutamate response was tested in the presence and absence of artificial cerebral spinal fluid.

In protocol one, responses to varying concentrations of direct glutamate administration did not differ between the sedentary and active animals. This is because heart rate responses were small and increases in LSNA also did not differ significantly between the active and sedentary animals. In protocol two, the initial injections of bicuculline increased the baseline blood pressure, heart rate, and LSNA in both the active and sedentary animals prior to any glutamate being injected. The increases did not differ significantly between the two groups. The glutamate injection responses at 5 and 15 minutes after the bicuculline was administered were greatly enhanced in the sedentary animals compared to the physically active animals. These enhancements were observed as increases in mean arterial pressure, however, were no longer observed by the 30- and 45-minute marks. Increases in LSNA were enhanced at the 5-, and 15-minute marks as well, however there was not a significant difference between the sedentary and active groups.  Once again, these effects were no longer observed at 30 and 45 minutes. This suggests that there was full recovery from bicuculline within the time course mentioned. Protocol three showed that responses to glutamate injections at different time points after injection of the bicuculline vehicle (aCSF) did not differ significantly from the control microinjections. In addition, responses to the repetitive microinjections did not differ significantly between the active and sedentary groups.

Overall, there were results that both confirmed and opposed the hypothesis. The hypothesis was confirmed in that there was enhancement in the response to glutamate in sedentary animals after the GABA receptor block was administered in the RVLM.  However, the LSNA responses did not differ between groups under direct glutamate activation nor under GABA receptor blockade conditions.

Based on the results, the paper determined several new findings that helped distinguish the effects of sedentary verses active conditions on nerve activity and blood pressure. First, sedentary conditions enhance GABAergic control of glutamate-sensitive neurons in the RVLM that regulate blood pressure. Second, sedentary conditions increase nerve activity when glutamate is administered in the absence of GABAergic modulation. Lastly, LSNA does not control the responses recorded in animals after GABA receptor blockers were administered.

This study helps us to possibly think about what other protocols we may want to add to the microinjection experiments. We could find other drugs that block glutamate or GABA responses in the RVLM and then see what effects present themselves when glutamate and GABA doses are given in the active and sedentary animals.

 

-Lyndsey M.