Electroacupuncture induces c-Fos expression in the rostral ventrolateral medulla and periaqueductal gray in cats: relation to opioid containing neurons

Abstract

Our previous studies have shown that electroacupuncture (EA) at the Neiguan–Jianshi (P5–P6) acupoints inhibits sympathetic outflow and attenuates excitatory visceral cardiovascular reflexes through enkephalin- or β-endorphin-related opioid receptors in the rostral ventrolateral medulla (rVLM). It is not known whether EA at these acupoints activates neurons containing enkephalin or β-endorphin in the rVLM as well as in the periaqueductal gray (PAG) that are involved in EA-mediated central neural regulation of sympathetic activity. The present study evaluated activated neurons in the rVLM and PAG by detecting c-Fos immunoreactivity, and identified the relationship between c-Fos nuclei and neuronal structures containing enkephalin or β-endorphin in these regions. To enhance the detection of cell bodies containing enkephalin or β-endorphin, colchicine (90–100 μg/kg) was injected into the subarachnoid space in anesthetized cats 28–30 h prior to EA or the sham-operated control for EA. Following bilateral barodenervation and cervical vagotomy, EA (1–4 mA, 2 Hz, 0.5 ms) was performed at the P5–P6 acupoints (overlying median nerve; n=7) for 30 min. Identical procedures, with the exception of electrical stimulation, were carried out in five control animals. EA decreased blood pressure (BP) in four of seven cats (5–15 mm Hg) while the sham procedure for EA produced no responses. Perikarya containing enkephalin were found in the rVLM and rarely in the PAG, while no cell bodies labeled with β-endorphin were identified in either region. Compared to animals in the control group, more c-Fos immunoreactivity, located principally in close proximity to fibers containing enkephalin or β-endorphin, was observed in the rVLM and ventrolateral PAG (vlPAG) in EA-treated cats. Moreover, neurons double-labeled with c-Fos and enkephalin in the rVLM were significantly increased in cats following EA stimulation ( P<0.05). These data indicate that EA at the P5–P6 acupoints activates neurons in the rVLM and vlPAG. These activated neurons contain enkephalin in the rVLM, and most likely interact with nerve fibers containing enkephalin or β-endorphin in both the rVLM and vlPAG. The results from this study provide the first anatomical evidence showing that EA at the P5–P6 acupoints has the potential to influence neuronal structures (perikarya, axons and/or dendrites) containing enkephalin or β-endorphin in specific regions of the brain stem. These neurons likely form the substrate for EA's influence on sympathoexcitatory cardiovascular reflexes.