Modulation of Neurally Mediated Vasodepression and Bradycardia by Electroacupuncture through Opioids in Nucleus Tractus Solitarius

Abstract

Stimulation of vagal afferent endings with intravenous phenylbiguanide (PBG) causes both bradycardia and vasodepression, simulating neurally mediated syncope. Activation of µ-opioid receptors in the nucleus tractus solitarius (NTS) increases blood pressure. Electroacupuncture (EA) stimulation of somatosensory nerves underneath acupoints P5-6, ST36-37, LI6-7 or G37-39 selectively but differentially modulates sympathoexcitatory responses. We therefore hypothesized that EA-stimulation at P5-6 or ST36-37, but not LI6-7 or G37-39 acupoints, inhibits the bradycardia and vasodepression through a µ-opioid receptor mechanism in the NTS. We observed that stimulation at acupoints P5-6 and ST36-37 overlying the deep somatosensory nerves and LI6-7 and G37-39 overlying cutaneous nerves differentially evoked NTS neural activity in anesthetized and ventilated animals. Thirty-min of EA-stimulation at P5-6 or ST36-37 reduced the depressor and bradycardia responses to PBG while EA at LI6-7 or G37-39 did not. Congruent with the hemodynamic responses, EA at P5-6 and ST36-37, but not at LI6-7 and G37-39, reduced vagally evoked activity of cardiovascular NTS cells. Finally, opioid receptor blockade in the NTS with naloxone or a specific μ-receptor antagonist reversed P5-6 EA-inhibition of the depressor, bradycardia and vagally evoked NTS activity. These data suggest that point specific EA stimulation inhibits PBG-induced vasodepression and bradycardia responses through a μ-opioid mechanism in the NTS.

Figure 1

Electroacupuncture (EA) at P5–6 attenuated PBG-induced decreases in mean arterial pressure (MAP) and heart rate (HR). Bar histograms display consistent responses to repeated administration of PBG every 10 min (Panels A). EA reduced Bezold Jarisch BP responses for 50 min and bradycardia for at least 70 min. Microinjection of saline into the NTS did not influence EA induced long-lasting inhibitory cardiac reflex responses (Panels B). Opioid receptor blockade with naloxone in NTS reversed EA modulation of depressor and bradycardia responses (Panels C). *Significant difference compared with control (before EA) responses to PBG. ^†Significant difference compared with preceding PBG reflex response during EA. Letters in (a–f) shown in the bars correspond to the original tracings showing decreases in MAP and HR (Panels c). Baseline BP and HR are shown above each bar as means ± SEM.

Figure 2

PBG-induced inhibitory cardiovascular reflex responses were examined with EA stimulation at specific points LI6–7, G37–39 or ST36–37. EA stimulation for 30 min at ST36–37 acupoints reduced the Bezold Jarisch reflex responses (Panel C) in contrast to LI6–7 (Panel A) and G37–39 (Panel B). Baseline BP and HR are shown above each bar as means ± SEM. *Significant difference compared with control responses prior to onset of EA.

Figure 3

Methods used to characterize NTS neurons are displayed in Panels A-E. Panels A and B respectively show frequency (coherence at frequency of 2.5 Hz) and time domain analyses demonstrating cardiac rhythmicity. Autospectra (AS) of BP and NTS activity and corresponding coherence function of 0.97. Neuronal activity of the NTS neuron is increased with intravenous phenylephrine (Panel C). Panel D displays the group data of NTS neuronal activity in response to administration of nitroprusside-evoked decrease in BP, phenylephrine-induced increased in BP, and PBG-induced decrease in BP. *Indicates significant difference in activity after administration of nitroprusside, phenylephrine or phenylbiguanide. NTS neurons also receive input from vagus and median nerves and to a lesser degree from cutaneous nerves like the superficial radial nerve underlying LI6-L7 acupoints (Panel E). Stimulation artifact is indicated with*. Neuronal discharge is displayed before and during stimulation of vagus, median or superficial radial nerve. PSD, power spectral density.

Figure 4

Bar histograms display vagus nerve-induced NTS neuronal activity without EA (Panel A) and with EA stimulation at P5–6 (Panels B and C). Iontophoresis of saline into the NTS did not influence the modulatory effect of EA (Panel B). *Indicates significant difference compared to activity prior to onset of EA (Panel B). Blockade of opioid receptor with iontophoresis of naloxone in the NTS reversed the EA effect (c) compared to vagal evoked activity prior to the blockade (b) for over 20 min (Panel C). Letters in a–e shown in the bars correspond with peristimulus histograms displayed to the right. *Indicates significant difference compared to control activity (a), while ^#shows significant difference compared to preceding (b) EA response. An example of recorded spike activity is shown above e.

Figure 5

Bar graphs display inhibition of vagally evoked activity of cardiovascular NTS neurons by EA at acupoints ST36–37, LI6–7 and G37–39. Panel A1 shows group data of NTS neuronal activity evoked by brief stimulation of vagus and acupoints at P5–6, ST36–37, LI6–7 and G37–39. Examples of NTS evoked activity by brief stimulation at P5–6 (a), ST36–37 (b) and LI6–7 (c) are shown in Panel A2. Peristimulus time histograms a, b, c correspond to letters in bars shown in Panel A1. Thirty-min EA application at ST36–37 inhibited vagally evoked NTS activity shown in Panel B1. Inset displays a vagally evoked NTS action potential. Panel B2 displays peristimulus histograms of NTS neuronal activity with letters a, b and c corresponding to the bar graph in Panel B1. In contrast to ST36–37 (Panel B1), EA at LI6–7 or G37–39 did not inhibit the vagally evoked NTS discharge (Panels C and D). (^†), *In Panel A1 indicate significant difference comparing LI6–7 and G37–39 evoked activity to vagally evoked discharge (^†) and to P5–6 and ST36–37 induced activity (*). *In Panel B1 indicates significant difference compared to activity before onset of EA.

Figure 6

Composite map displays sites of microinjections, iontophoresis and extracellular recordings in intermediate NTS of cats. For ease of representation, all recording sites are displayed on the right and microinjections on the left. (*), sites located within NTS. O, control site outside intermediate NTS. Coronal section is 0 to 0.6 mm rostral to obex.