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

Abstract

Overactivity of the sympathetic nervous system (SNS) is a hallmark of many cardiovascular diseases. It is also well-known that physical inactivity independently contributes to cardiovascular diseases, likely in part via increased SNS activity. Recent work from our laboratory has demonstrated increased SNS responses in sedentary animals following either direct activation or disinhibition of the rostral ventrolateral medulla (RVLM), an integral cardiovascular brainstem region. These data led us to hypothesize that the interaction between excitation and inhibition of the RVLM is altered in sedentary versus physically active animals. To test this hypothesis, we recorded mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA) in Inactin anesthetized rats that were housed for 8-12 weeks with or without access to a running wheel. Pressor responses to direct activation of the RVLM with glutamate were similar between groups under intact conditions. However, blockade of γ-aminobutyric acid (GABA)(A) receptors with bicuculline selectively enhanced pressor responses to glutamate in sedentary animals. Interestingly, LSNA responses to glutamate were not enhanced in sedentary versus active animals in the presence or absence of tonic GABAergic tone. These results suggest that sedentary compared to active conditions enhance GABAergic inhibition of glutamate-sensitive neurons in the RVLM that are involved in blood pressure regulation, and by mechanisms that do not involve LSNA. We also speculate that regular physical activity has differential effects on SNS activity to specific vascular beds and may reduce the risk of developing cardiovascular diseases via changes occurring in the RVLM.

Keywords: GABA; blood pressure; brainstem; exercise; glutamate.

Figure 1

Peak mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (LSNA) responses to activation of the rostral ventrolateral medulla with increasing concentrations of glutamate (30 nl) in sedentary (filled circles, n = 16) and physically active rats (open circles, n = 17). There was a significant effect of glutamate concentration to increase MAP, HR, and LSNA (^*p < 0.05) but responses did not differ between groups.

Figure 2

Representative examples of arterial pressure (AP) and LSNA responses to unilateral microinjections of glutamate (30 nl, 10 mM) into the rostral ventrolateral medulla before and after microinjection of the GABA_A antagonist bicuculline (60 nl, 5 mM). (A) Control response to glutamate in one sedentary rat. (B) Response to glutamate in the same sedentary rat 5 min after bicuculline. (C) Control response to glutamate in one physically active rat. (D) Response to glutamate in the same physically active rat 5 min after bicuculline. Note enhanced pressor response only in sedentary rat after bicuculline and enhanced LSNA response in both sedentary and physically active rats following bicuculline. Arrow represents timing of glutamate microinjection. Abbreviations are as defined in Figure 1.

Figure 3

Peak MAP, HR, and LSNA responses to activation of the rostral ventrolateral medulla with glutamate (10 mM, 30 nl) in the presence or absence of GABA_A receptor blockade with bicuculline (5 mM, 60 nl). Sedentary (filled bars, n = 10) and physically active rats (open bars, n = 10) had similar pressor and sympathoexcitatory response to glutamate prior to bicuculline (Glu Ctrl). Following bicuculline, sedentary animals exhibited a selective enhancement in pressor responses to glutamate at 5 and 15 min (^*p < 0.05), resulting in significantly greater pressor responses compared to physically active animals (^#p < 0.05 at 5 and 15 min). LSNA response were similarly enhanced in both groups at 5 and 15 min (^*with bracket, p < 0.05). Responses in both groups returned to control within 30 min. The small HR responses were also significantly different between groups (p < 0.05). Abbreviations are as defined in Figure 1.

Figure 4

Peak MAP, HR, and LSNA responses to activation of the rostral ventrolateral medulla with glutamate (10 mM, 30 nl) in the presence or absence of artificial cerebrospinal fluid injections (aCSF, 60 nl). aCSF had no significant effect on responses in either sedentary (filled bars, n = 8) or physically active rats (open bars, n = 7) over the entire time course of injections and there were no significant differences between groups. Abbreviations are as defined in Figure 1.

Figure 5

Histological representation of microinjection sites plotted on brainstem sections that have been modified from a standard rat atlas (Paxinos and Watson, 2007). Unilateral injection sites from sedentary (filled circles, n = 16) and physically active animals (open circles, n = 16) are plotted on opposite sides for clarity. All microinjection sites were associated with pressor and sympathoexcitatory responses to glutamate; were confined within the general boundaries of the rostral ventrolateral medulla (RVLM); and were distributed similarly between groups. FN, facial nucleus; NA, nucleus ambiguus; Py, pyramidal tract; SP5, spinal trigeminal tract.