Neurogenic Hypotension and Bradycardia Modulated by Electroacupuncture in Hypothalamic Paraventricular Nucleus

Abstract

Electroacupuncture (EA) stimulates somatic median afferents underlying P5-6 acupoints and modulates parasympathoexcitatory reflex responses through central processing in the brainstem. Although decreases in blood pressure and heart rate by the neural-mediated Bezold-Jarisch reflex responses are modulated by EA through opioid actions in the nucleus tractus solitarius and nucleus ambiguus, the role of the hypothalamus is unclear. The hypothalamic paraventricular nucleus (PVN) is activated by sympathetic afferents and regulates sympathetic outflow and sympathoexcitatory cardiovascular responses. In addition, the PVN is activated by vagal afferents, but little is known about its regulation of cardiopulmonary inhibitory hemodynamic responses. We hypothesized that the PVN participates in the Bezold-Jarisch reflex responses and EA inhibits these cardiopulmonary responses through the PVN opioid system. Rats were anesthetized and ventilated, and their heart rate and blood pressures were monitored. Application of phenylbiguanide every 10 min close to the right atrium induced consistent depressor and bradycardia reflex responses. Unilateral microinjection of the depolarization blockade agent kainic acid or glutamate receptor antagonist kynurenic acid in the PVN reduced these reflex responses. In at least 70% of the rats, 30 min of bilateral EA at P5-6 acupoints reduced the depressor and bradycardia responses for at least 60 min. Blockade of the CCK-1 receptors converted the non-responders into EA-responders. Unilateral PVN-microinjection with naloxone reversed the EA inhibition. Vagal-evoked activity of the PVN cardiovascular neurons was reduced by 30 min EA (P5-6) through opioid receptor activation. These data indicate that PVN processes inhibitory cardiopulmonary reflexes and participates in EA-modulation of the neural-mediated vasodepression and bradycardia.

Keywords: CCK; opioids; parasympathetic; serotonin; somatosensory.

FIGURE 1

Repeated administration of PBG induced consistent Bezold-Jarisch reflex responses. The depressors and bradycardia reflex responses were not altered after microinjection of saline into the PVN. Baselines blood pressures and heart rates in mean ± SEM shown above bars were not significantly different throughout experiments.

FIGURE 2

Electroacupuncture (EA) at P5-6 acupoints reduced the Bezold-Jarisch responses evoked by PBG every 10 min. EA reduced the inhibitory cardiovascular responses for at least 60 min (A,B). Blockade of opioid receptors with naloxone in the PVN reversed the inhibitory effects of EA on cardiopulmonary reflex responses (B) in contrast to saline (A). Baselines blood pressures and heart rates in mean ± SEM shown above bars were not significantly different throughout experiments. *Significant difference compared with control PBG response. ^†Significant difference from the preceding PBG response.

FIGURE 3

The role of CCK in the effects of EA during hypotensive and bradycardia responses. PVN microinjection of the agonist CCK-8 inhibited the effect of EA modulation of the cardiopulmonary responses. Baselines blood pressures and heart rates in mean ± SEM shown above bars were not significantly different throughout experiments. *Significant difference compared with control PBG response. ^†Significantly different compared with the preceding PBG response.

FIGURE 4

Rats non-responsive to EA at the P5-P6 acupoint (overlying the median nerve) were tested with second EA treatment to modulate the PBG-induced Bezold-Jarisch reflex responses. Vehicle control and saline control do not affect the lack of responsiveness to EA in the non-responders.

FIGURE 5

Non-responders to EA on the Bezold-Jarisch responses were converted into responders following blockade of the CCK₁ receptor in the PVN. Unlike the responders, EA inhibition of the depressor and bradycardia was not observed in the non-responders. However, repeat EA reduced the depressor and bradycardia after microinjection of devazepide (the CCK antagonist) into the PVN in the non-responder (A). The effectiveness of EA, after CCK1 blockade, was reversed with naloxone (B). An individual set of data on blood pressure (A–E) and heart rate (F–J) tracings is displayed below the bar histograms. The letters above the tracings correspond with the letters shown in the bars. *p < 0.05, indicates significant reduction in heart rate and blood pressure during effects of EA compared with PBG responses before EA. ^†p < 0.05, indicates significant reversal of EA effect following naloxone compared with PBG response prior to blockade.

FIGURE 6

Methods used to classify paraventricular nucleus (PVN) neurons that received input from the vagal nerve, the median nerve, and the baroreceptor. (A) Displays strong correlation and coherence between PVN neuronal activity and blood pressure. (B) Shows time domain analysis of a strong relationship between PVN discharge and blood pressure. (C) Displays recording of blood pressure (BP) and activity of PVN neuron (receiving vagal input) after intravenous nitroprusside. Group data of PVN neuronal activity following administration of nitroprusside and phenylephrine (D). (E) Shows with a bar histogram the evoked activity in the PVN following PBG intravenous administration. (F) Displays consistent evoked PVN activity by repeated cervical vagal nerve stimulation. The inlay displays time of vagal stimulation and its evoked spike in PVN (F). *Indicates significant difference in PVN activity following administration of nitroprusside or phenylephrine (D) or PBG (E).

FIGURE 7

Repeated cervical vagus nerve stimulation evoked PVN activity that was reduced by EA for at least 70 min. Saline microinjection did not influence the EA inhibition (A). In contrast, blockade of opioid receptors in the PVN reversed the EA action (Bar C) followed by the long-lasting EA effect for another 30 min (B). Peristimulus histograms (A–C) display discharge activity of a cardiovascular PVN neuron with input from vagal, baroreceptor, and EA-activated median nerves. The letters A, B, and C in the peristimulus histograms correspond with the letters in the bars. *Significant difference compared with control PBG response. ^†Significant difference from the preceding vagal-evoked activity.

FIGURE 8

A composite map displays sites of microinjection and recording in the PVN in rats. For ease of presentation, extracellular recording sites in the PVN are displayed on the left (+) and microinjections sites (*) on the right. PVN, paraventricular nucleus; AH, anterior hypothalamic nucleus; 3V, third ventricle.

FIGURE 9

A diagram displays the afferent (the solid red arrow) and efferent (the dashed red arrow) nerves and projections involved in phenylbiguanide (PBG) induced cardiopulmonary reflex response modulated by electroacupuncture (EA). For ease of display, on the right is the PBG-related neuronal pathway (red arrows), while on the left is the reversal of the evoked activities, leading to reduction of hypotension and bradycardia by EA inhibition (blue arrows). The dialogue box on the left shows opioid and CCK systems observed in non-responders. The opioid receptor is shown as a blue star and the CCK receptor as a red triangle. PVN, paraventricular nucleus; NTS, nucleus tractus solitaries; rVLM, rostral ventrolateral medulla; NA, nucleus ambiguus. (+), excitatory projection. (–), inhibitory projection. NTS-rVLM, inhibitory projection through GABAergic cVLM neurons. NA-Heart, inhibitory projection through acetylcholinergic neurotransmission.