Reflex Inhibition of Heart Rate and Efferent Cardiac Sympathetic Outflow Induced by Colorectal Distension in Anesthetized Rats

Neurons in the nucleus tractus solitarius mediate the acupuncture analgesia in visceral pain rats

The study investigated the role of nucleus tractus solitarius (NTS) neurons in electroacupuncture (EA) analgesia in colorectal distension (CRD) rats. NTS neurons responding to both CRD test and EA conditioning stimulations were considered somato-visceral convergent neurons. The neuronal activities evoked by graded CRD showed multiple firing patterns indicating multisynaptic connections. Some of the CRD excitatory neurons were inhibited by EA and vice versa. There was no discrepancy among different acupoints in inducing the changes of unit discharges. Conclusively, EA could regulate CRD related neurons in the NTS through polysynaptic cross-talk mechanism, which mediates EA analgesia on visceral pain in anesthetized rats.

Mineralocorticoid and glucocorticoid receptors in the amygdala regulate distinct responses to colorectal distension. Neuropharmacology. 2009;56:514-521. [PMID: 18977372 DOI: 10.1016/j.neuropharm.2008.10.004]

Previously we found that exposure of the amygdala to elevated levels of corticosterone (CORT) induces anxiety-like behavior coupled to colonic hypersensitivity to distension, however, the specific corticoid receptor mediating the CORT responses remains controversial. In this study we investigated, through the use of selective antagonists, the relative role of amygdaloid mineralocorticoid (MR) versus glucocorticoid receptors (GR) in CORT-mediated spinal and cardiovascular pseudoaffective reflex responses to colorectal distension (CRD). Micropellets containing, CORT and a selective MR antagonist (spironolactone) or GR antagonist (mifepristone) were implanted stereotaxically onto the dorsal margin of the amygdala in rats. On day 7 post-implantation in response to graded CRD we measured: (i) changes in the electrical activity of dorsal horn neurons in the L6-S1 spinal cord and (ii) the cardiovascular depressor responses. Exposure of the amygdala to CORT-releasing micropellets increased the proportion of spinal neurons showing high-threshold and/or long-lasting responses and potentiated the magnitude of excitation. Elevated levels of amygdala CORT also increased the magnitude of the cardiovascular depressor response to CRD. MR but not GR antagonism prevented the increase in spinal cord neuronal excitation, whereas either the MR or GR antagonist decreased the magnitude of the depressor cardiovascular response to CRD. We conclude that MR in the amygdala trigger descending pathways facilitating viscero-nociceptive processing in the spinal cord, whereas MR and GR have a non-redundant role in CORT-induced potentiation of the autonomic pseudoaffective responses to colorectal stimuli.

Neural mechanism of bradycardiac responses elicited by acupuncture-like stimulation to a hind limb in anesthetized rats

The effects of acupuncture-like stimulation of a hind limb on heart rate were examined in anesthetized rats. An acupuncture needle, having a diameter of either 160 or 340 microm, was inserted into the skin and underlying muscles at a depth of about 5 mm and twisted right and left twice every second for 1 min. Stimulation by a needle with a diameter of either 160 or 340 microm produced a decrease in heart rate. Severance of the femoral and sciatic nerves ipsilateral to the hind-limb stimulation completely abolished the bradycardiac response. Also, heart rate was significantly decreased by acupuncture-like stimulation of the hind-limb muscles alone, but was not significantly influenced by the stimulation of the hind-limb skin alone. The bradycardiac response induced by acupuncture-like stimulation was not influenced by bilateral severance of the vagal nerves at the cervical level, but was abolished by bilateral stellectomy. Acupuncture-like stimulation of the hind limb induced a decrease in the activity of the cardiac sympathetic efferent nerve as well as a decrease in heart rate. These results indicate that the decrease in heart rate induced by acupuncture-like stimulation of a hind limb is a reflex response. The afferent pathway is composed of hind-limb muscle afferents, and the efferent pathway is composed of cardiac sympathetic nerves.