Spinal nociceptin mediates electroacupuncture-related modulation of visceral sympathoexcitatory reflex responses in rats

Abstract

The role of nociceptin and its spinal cord neural pathways in electroacupuncture (EA)-related inhibition of visceral excitatory reflexes is not clear. Nociceptin/orphanin FQ (N/OFQ) is an endogenous ligand for a G protein-coupled receptor, called the N/OFQ peptide (NOP) receptor, which has been found to be distributed in the spinal cord. The present study investigated the importance of this system in visceral-cardiovascular reflex modulation during EA. Cardiovascular pressor reflex responses were induced by gastric distension in Sprague-Dawley rats anesthetized by ketamine and xylazine. An intrathecal injection of nociceptin (10 nM) at T1-2 attenuated the pressor responses by 35%, similar to the influence of EA at P 5-6 (42% decrease). An intrathecal injection of the NOP antagonist, [N-Phe(1)]nociceptin(1-13) NH(2), partially reversed the EA response. Pretreatment with the opioid receptor antagonist naloxone did not alter the EA-like inhibitory effect of nociceptin on the pressor reflex, whereas a combination of nociceptin receptor antagonist with naloxone completely abolished the EA response. An intrathecal injection of nociceptin attenuated the pressor responses to the electrical stimulation of the rostral ventrolateral medulla by 46%, suggesting that nociceptin can regulate sympathetic outflow. Furthermore, a bilateral microinjection of NOP antagonist into either the dorsal horn or the intermediolateral column at T1 partially reversed the EA inhibitory effect. These results suggest that nociceptin in the spinal cord mediates part of the EA-related modulation of visceral reflex responses.

Fig. 1.

Gastric distension response to nociceptin in the spinal cord. A: blood pressure tracings of an individual animal, represented by a, b, and c, are displayed above representative bar histograms. Small arrows on tracings indicate time of gastric distension. B: effect of nociceptin injected intrathecally at T1–2 (long arrow) on mean arterial pressure (MAP) responses to gastric distension. C: effect of naloxone and nociceptin/orphanin FQ peptide receptor antagonist on nociceptin-induced attenuation of pressor responses induced by gastric distension. Baseline blood pressures (in mmHg) before distension are indicated below each bar. Gastric distension was repeated every 10 min. Bars show increase in MAP (±SE) induced by distension of the stomach. *P < 0.05, significant difference after intrathecal injection of nociceptin. N, number of animals.

Fig. 2.

Influence of opioid antagonism on nociceptin response. A: effect of electroacupuncture (EA) on reflex blood pressure responses. B: blood pressure responses to intrathecal injection of nociceptin receptor antagonist after onset of EA. Labels a–d on the bars correspond to the tracings shown above the bars. C: blood pressure responses to intrathecal injection of both nociceptin and opioid receptor antagonist during EA. *P < 0.05, significant difference compared with pre-EA. N, number of animals.

Fig. 3.

Spinal nociceptin system in cardiovascular responses to electrical stimulation in the rostral ventrolateral medulla (rVLM). A: effect of intrathecal injection of nociceptin on the BP responses to electrical stimulation of the rVLM (bars). *P < 0.05, significant difference after intrathecal injection of nociceptin. B: composite map displaying sites of insertion of stimulation electrode (*) in the rVLM. All insertions were unilateral (side chosen randomly). Sections are 2.0–2.5 and 2.5–3.0 mm caudal to the interaural line (Ref. 33). Py, pyramidal tract; Amb, ambiguous nucleus; 7, facial nucleus; Sp5, spinal trigeminal nucleus; ION, inferior olivary nucleus; N, number of animals.

Fig. 4.

Spinal neural pathway of nociceptin in EA modulation. Microinjection of nociceptin/orphanin FQ peptide antagonist in the dorsal horn (DH; A) or the intermediolateral column (IML; B) at T1 following 20 min of EA. *P < 0.05. Bars represent pressor responses to gastric distension. C: composite map displaying sites of microinjections in the DH and the IML. N, number of animals.