Serotonergic projections from nucleus raphe pallidus to rostral ventrolateral medulla participate in acupuncture modulation of cardiovascular excitatory reflexes

Abstract

Somatic afferent stimulation during electroacupuncture (EA) at P5–6 (over median nerves) inhibits pressor reflexes. The present study examined the role of nucleus raphe pallidus (NRP) and its synaptic input on neurons in the rostral ventrolateral medulla (rVLM) during EA. We have shown that EA activates NRP serotonergic (5HT) neurons. Since 5HT NRP neurons are known to project to the rVLM, we hypothesized that the NRP facilitates EA inhibition of cardiovascular sympathoexcitatory reflex responses through a 5HT1A mechanism in the rVLM. Heart rate and blood pressure (BP) were monitored in anesthetized and ventilated cats. Bradykinin application on the gallbladder (GB) induced consistent pressor reflexes averaging 39±01 mmHg in 5 cats. EA reduced the BP responses from 36±4 to 17±8 mmHg in 5 cats. Inactivation of NRP with kainic acid (1 mM, 50 nl) in 4 animals immediately reversed the EA-induced reflex attenuation from 23±4 to 43±9 mmHg. 5HT1A receptor blockade in the rVLM with WAY-100635 (1 mM, 75 nl) reversed the EA BP inhibition from 17±3 to 31±5 mmHg in 4 cats. In the absence of EA, NRP injection of DLH (to mimic EA) reduced the GB BP reflex response from 40±8 to 19±5 mmHg. Likewise, 5HT1A receptor blockade in the rVLM reversed the DLH NRP-evoked BP inhibition from 19±5 to 33±7 mmHg in 4 cats. Thus, activation of 5HT neurons in the NRP during EA-somatic stimulation attenuates sympathoexcitatory responses through 5HT1A receptor activation in the rVLM. Supported by HL-63313 and HL-72125.