Nucleus Ambiguus Processing of Electroacupuncture Cardiovascular Inhibitory Reflex Responses: Role of Opioids and GABA

Abstract

Stimulation of cardiopulmonary receptors with phenylbiguanide (PBG) elicits cardiovascular reflex responses, including decreases in blood pressure and heart rate mediated by the brainstem parasympathetic cardiac neurons in the nucleus ambiguus (NA). The present study examined the NA mechanisms underlying the influence of electroacupuncture (EA) on the PBG‐induced hypotension and bradycardia. We hypothesized that somatic stimulation during EA modulates PBG responses through opioid and γ‐aminobutyric acid (GABA) mechanisms in the NA. Anesthetized and ventilated cats were studied during repeated stimulation with PBG or cervical vagus while low frequency EA (2 Hz) was applied at P5‐6 overlying the median nerve for 30 min and NA neuronal activity, heart rate and blood pressure were recorded. Microinjection of kainic acid into the NA attenuated the PBG‐induced bradycardia from 67±17 to 46±14 bpm. EA likewise reduced the PBG‐induced depressor and bradycardia reflex by 39 and 68%, respectively. Cervical vagus evoked activity in the NA was reduced rapidly by EA for at least 40 min. Blockade of opioid or GABAA receptors using naloxone and gabazine reversed the EA‐related modulation of the evoked vagal activity by 73 and 53%, respectively. Thus, EA at P5‐6 decreases PBG evoked hypotension and bradycardia as well as the vagus evoked NA activity through opioid and GABA systems. Supported by HL‐63313.