Repetitive Electroacupuncture Attenuates Cold-Induced Hypertension through Enkephalin in the Rostral Ventral Lateral Medulla

This experimental study investigated the effects of repetitive electroacupuncture on hypertension and its underlying neurobiological mechanisms, using a clinically relevant animal model of cold-induced hypertension. Researchers at the University of California developed a protocol that simulates real clinical conditions, in which 54 rats were exposed to cold (6°C) for six weeks to induce sustained hypertension, similar to human essential hypertension. After hypertension developed, the animals were randomized into electroacupuncture, sham-electroacupuncture, untreated hypertensive control, and normotensive control groups. The electroacupuncture protocol consisted of bilateral stimulation of points ST-36/ST-37 (Zusanli-Shangjuxu) with specific parameters: 2 Hz frequency, 0.1-0.4 mA intensity, 30-minute duration, applied twice a week for five weeks.

These points were chosen because they are located over the deep peroneal nerve and are widely used clinically for cardiovascular conditions. The results demonstrated that electroacupuncture was highly effective in reducing elevated blood pressure. After six treatment sessions, a significant reduction was observed in both systolic and diastolic blood pressure, with effects sustained throughout the treatment period. Notably, the antihypertensive effect persisted for at least 72 hours after the last session, demonstrating a clinically relevant prolonged action.

This persistence of effects is particularly important because it suggests that electroacupuncture can offer lasting benefits with less frequent application. To understand the neurobiological mechanisms, the researchers focused on the rostral ventrolateral medulla (rVLM), a critical brain region for the control of sympathetic activity and blood pressure regulation. Through real-time PCR techniques, electroacupuncture was found to significantly increase preproenkephalin mRNA expression in the rVLM 72 hours after treatment. Preproenkephalin is the precursor of enkephalins, endogenous opioid neurotransmitters that play a fundamental role in cardiovascular modulation.

Complementary pharmacologic experiments using specific microinjections in the rVLM confirmed the role of delta-opioid receptors in these effects. Administration of the delta-opioid antagonist ICI 174,864 in animals treated with electroacupuncture partially reversed the antihypertensive effects, while stimulation with the agonist DADLE in sham-group animals reproduced the hypotensive effects of electroacupuncture. These findings establish a clear mechanistic pathway: electroacupuncture stimulates enkephalin synthesis in the rVLM, which acts through delta-opioid receptors to reduce sympathoexcitatory neuronal activity, resulting in decreased blood pressure. The cold-induced hypertension model used has particular clinical relevance because it simulates natural conditions associated with the development of human essential hypertension, including increased sympathetic activity.

Epidemiologic studies demonstrate a higher prevalence of hypertension in cold regions and during colder seasons, validating this experimental model. The clinical implications are significant, suggesting that electroacupuncture may represent a viable nonpharmacologic approach for the treatment of chronic hypertension, with well-defined mechanisms of action involving the endogenous opioid system.