Electroacupuncture Improves Baroreflex and γ-Aminobutyric Acid Type B Receptor-Mediated Responses in the Nucleus Tractus Solitarii of Hypertensive Rats

Abstract

Electroacupuncture (EA) has been reported to benefit hypertension, but the underlying mechanisms are still unclear. We hypothesized that EA attenuates hypertension, in part, through modulation of γ-aminobutyric acid (GABA) receptor function in the nucleus tractus solitarii (NTS). In the present study, the long-term effect of EA on GABA receptor function and expression was examined in the NTS of two-kidney, one-clip (2K1C) renovascular hypertensive rats. EA (0.1-0.4 mA, 2 and 15 Hz) was applied at Zusanli (ST36) acupoints overlying the deep fibular nerve for 30 min once a day for two weeks. The results showed that long-term EA treatment improved blood pressure (BP) and markedly restored the baroreflex response in 2K1C hypertensive rats. The increased pressor and depressor responses to microinjection of GABA_B receptor agonist and antagonist into the NTS in the hypertensive rats were blunted by the EA treatment. Moreover, EA treatment attenuated the increased GABA_B receptor expression in the NTS of hypertensive rats. In contrast, EA had no significant effect on the GABA_A receptor function and expression in the NTS of 2K1C hypertensive rats. These findings suggest that the beneficial effects of EA on renovascular hypertension may be through modulation of functional GABA_B receptors in the NTS.

Figure 1

Long-term EA treatment attenuates hypertension in 2K1C hypertensive rats. (a) Time course of mean arterial pressure (MAP) in each group over 27 days. (b) Time course of heart rate (HR) in each group over 27 days. MAP and HR were measured by radiotelemetry. (c) The change in the urinary norepinephrine excretion at day 27 after the 2K1C hypertensive model was established. Data are presented as mean ± SE; n = 7–9 rats; ^∗P < 0.05, ^∗∗P < 0.01 vs. the sham group; ^#P < 0.05, ^##P < 0.01 vs. 2K1C group.

Figure 2

Long-term EA treatment restores the baroreflex response in 2K1C hypertensive rats. (a) Grouped heart rate (HR) baroreflex response to each 10 mmHg change in mean arterial pressure (MAP) elicited by phenylephrine. (b) Grouped heart rate (HR) baroreflex response to each 10 mmHg change in mean arterial pressure (MAP) evoked by sodium nitroprusside. (c–e) Spontaneous baroreflex analysis with overall baroreflex gain (combination of up and down sequences), up sequence gain, and down sequence gain (in ms/mmHg). Data are presented as mean ± SE; n = 7–9 rats; ^∗P < 0.05 vs. the sham group; ^#P < 0.05 vs. the 2K1C group.

Figure 3

Identification of the microinjection sites in the nucleus tractus solitarii (NTS). (a) A representative photomicrograph showing the NTS microinjection site and the location of this microinjection site based on the rat brain atlas. The arrow indicates the injection site. (b) Functional identification of the NTS with microinjections of glutamate (Glu; 300 pmol in 50 nL) in one rat. Unilateral glutamate produced decreases in arterial pressure (AP) and HR.

Figure 4

Effect of the GABA_B receptor antagonist, CGP-35348, microinjected into the NTS on MAP and HR in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. (a) Representative original tracings showing AP and HR changes evoked by microinjection of CGP-35348 (100 pmol in 50 nL) into the NTS in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. The horizontal bar represents recording duration of 2 min. Arrows indicates the injection of CGP-35348 (CGP). The peak alteration (Δ) of MAP (b) and peak alteration (Δ) of HR (c) after microinjection of CGP-35348 (100 pmol in 50 nL) into the NTS of the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. Values are means ± SE (n = 7–8 in each group). ^∗P < 0.05 vs. sham group; #P < 0.05 vs. 2K1C group.

Figure 5

Effect of the GABA_B receptor agonist baclofen on MAP and HR in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. (a) Representative original tracings showing AP and HR changes evoked by microinjection of baclofen (50 pmol in 50 nL) into the NTS in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. The peak alteration (Δ) of MAP (b) and peak alteration (Δ) of HR (c) after microinjection of baclofen (50 pmol in 50 nL) into the NTS. Values are means ± SE (n = 7–8 in each group). ^∗P < 0.05 vs. sham group; #P < 0.05 vs. 2K1C group.

Figure 6

Effect of the GABA_A receptor antagonist bicuculline and agonist muscimol on MAP and HR in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA rats. MAP (a) and HR (b) changes evoked by microinjection of bicuculline (10 pmol in 50 nL) into the NTS. MAP (c) and HR (d) changes evoked by microinjection of muscimol (100 pmol in 50 nL) into the NTS. Values are means ± SE (n = 7 or 5 rats in each group). ^∗P < 0.05 vs. saline control in each group.

Figure 7

Effect of long-term EA treatment on GABA_B and GABA_A receptor expressions in the NTS of rats. (a, b) GABA_B receptor (GBR) and GABA_A receptor (GAR) mRNA levels within the NTS in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups which were detected with real-time RT-PCR. Data were normalized with 18 S rRNA. (c, d) Quantitative analysis of GABA_B receptor (GBR) and GABA_A receptor (GAR) protein levels within the NTS in the sham, 2K1C, 2K1C + EA, and 2K1C + SEA groups. The upper panel shows the representative immunoblots of GABA_B receptor (GBR) and GABA_A receptor (GAR) protein levels within the NTS in each group. Values are normalized using β-actin. Values are means ± SE (n = 6 in each group). ^∗P < 0.05 vs. the sham group; ^#P < 0.05 vs. the 2K1C group.