Inhibition of nociceptive responses of spinal cord neurones during hypertension involves the spinal GABAergic system and a pain modulatory center located at the caudal ventrolateral medulla

Hypertension and Postoperative Pain: A Prospective Observational Study

Objectives

The relationship between pain and hypertension is of great pathophysiological and clinical interest in the pain field, but the mechanism is poorly understood. This study used the postoperative patient-controlled analgesia (PCA) dose and the visual analysis scale (VAS) score to assess the relationship between pain and hypertension.

Methods

In this prospective study in a single-center hospital, 200 participants were enrolled and divided into three groups: normotensive group, hypertension without treatment group, and hypertension with treatment group. The participants scheduled for elective inhalational general anesthesia were interviewed at hospital admission.

Results

A significant difference was observed in analgesic dosage on postoperative days 1, 2, and 3 between the female normotensive group and female hypertension with treatment group (independent-samples, one-way analysis of covariance, age, and weight as covariates:P=0.021, 0.014, 0.032). No significant differences in the VAS scores and PCA dosages were observed between the male normotensive group and any one of the male hypertensive groups.

Conclusion

We agree that hypertensive hypoanalgesia exists in some experimental settings. The mechanism linking postoperative pain and hypertension is far more complex than we initially believed. Therefore, more studies are required to investigate the roles that antihypertensive drugs, sex, and psychological stress play. Antihypertensive drugs may play a crucial role in mediating the relationship between pain and hypertension. Psychosocial factors were discussed but were not examined.

Pain modulation induced by respiration: phase and frequency effects

The voluntary control of respiration is used as a common means to regulate pain and emotions and is fundamental to various relaxation and meditation techniques. The aim of the present study was to examine how breathing frequency and phase affect pain perception, spinal nociceptive activity (RIII-reflex) and brain activity (scalp somatosensory-evoked potentials - SEP's). In 20 healthy volunteers, painful electric shocks individually adjusted to 120% of the RIII-reflex threshold were delivered to the sural nerve near the end of inspiration or expiration phases, during three cued-breathing conditions: (1) slow breathing (0.1 Hz) with slow (4s) inspiration (0.1Hz-SlowIns), (2) slow breathing (0.1 Hz) with fast (2s) inspiration (0.1 Hz-FastIns), and (3) normal breathing (0.2 Hz) with fast (2s) inspiration (0.2 Hz). Pain ratings were not affected by breathing patterns (p=0.3), but were significantly lower during inspiration compared with expiration (p=0.02). This phase effect was also observed on the N100 component of SEP's, but only in the 0.1-Hz-FastIns condition (p=0.03). In contrast, RIII-reflex amplitude was greater during inspiration compared with expiration (p=0.02). It was also decreased in the 0.1-Hz-SlowIns compared with the 0.2-Hz condition (p=0.01). Slow breathing also increased the amplitude of respiratory sinus arrhythmia (RSA), although these changes were not significantly associated with changes in pain responses. In conclusion, this study shows that pain and pain-related brain activity may be reduced during inspiration but these changes are dissociated from spinal nociceptive transmission. The small amplitude of these effects suggests that factors other than respiration contribute to the analgesic effects of relaxation and meditation techniques.

Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

N-methyl-D-aspartate receptors (NMDAR) have a role in cardiovascular control at the nucleus tractus solitarii (NTS), eliciting increases or decreases in blood pressure (BP), depending on the area injected with the agonists. In spite of the association between cardiovascular control and pain modulation, the effects of manipulating NMDAR in pain responses have never been evaluated. In this study, we decreased the expression of NMDAR in the NTS using gene transfer to target receptor subunits and evaluate long-term effects. Seven days after the injection of lentiviral vectors containing the NR1a subunit cDNA of NMDAR, in antisense orientation, into the intermediate NTS of Wistar rats, BP was measured, and the formalin test of nociception was performed. The antisense vector induced a decrease of NR1 expression in the NTS and elicited BP rises and hypoalgesia. Antisense vectors inhibited formalin-evoked c-Fos expression in the spinal cord, indicating decreased nociceptive activity of spinal neurons. Using a time-course approach, we verified that the onset of both the increases in BP and the hypoalgesia was at 4 days after vector injection into the NTS. The injection of NMDA into the NTS reversed the effects of antisense vectors in pain behavioral responses and spinal neuronal activation and decreased BP and heart rate. The present study shows that the NR1 subunit of the NMDAR at the NTS is critical in the regulation of tonic cardiovascular and nociceptive control and shows an involvement of the nucleus in the modulation of sustained pain.

Does chronic pain alter the normal interaction between cardiovascular and pain regulatory systems? Pain modulation in the hypertensive-monoarthritic rat

Hypertension-associated hypoalgesia is widely recognized in acute pain conditions. In chronic pain states, however, the relationship between blood pressure and pain sensitivity is still ill-defined, with different authors reporting negative, positive, or even no relationship at all. This work addresses this issue, using complete Freund's adjuvant (CFA)-induced monoarthritis in different models of hypertension: Spontaneous (spontaneously hypertensive rats, SHR), induced by infusion of angiotensin II (ANG) or 1,3-dipropyl-8-sulfophenylxanthine (DPSPX, an adenosine receptors' antagonist), and renal artery ligation (RAL). Nociceptive responses associated with monoarthritis were evaluated by different behavioral tests (von Frey, ankle-bend and CatWalk) and by quantification of Fos expression at the dorsal horn upon noxious stimulation. In all hypertension models, higher von Frey thresholds and lower Fos expression were detected in hypertensive rats with chronic inflammatory pain, as compared to normotensive monoarthritic rats. In SHR and DPSPX, but not ANG or RAL models, hypertensive animals displayed lower inflammation than normotensives. Ankle-bend and CatWalk results indicated lower pain sensitivity in hypertensive rats only in SHR and DPSPX models. The present study shows the importance of using multiple models of hypertension, and evaluating pain responses by various methods, to better understand the complexity of the interactions between pain and cardiovascular regulatory systems.

PERSPECTIVE

This study used different models of hypertension to investigate whether chronic pain alters the normal integration of cardiovascular and pain regulatory systems. A complete understanding of the mechanisms underlying the complex interactions between these systems may disclose future therapeutic approaches to treat hypertension/chronic pain comorbidity states.

Microinjection of angiotensin II in the caudal ventrolateral medulla induces hyperalgesia

Nociceptive transmission from the spinal cord is controlled by supraspinal pain modulating systems that include the caudal ventrolateral medulla (CVLM). The neuropeptide angiotensin II (Ang II) has multiple effects in the CNS and at the medulla oblongata. Here we evaluated the expression of angiotensin type 1 (AT(1)) receptors in spinally-projecting CVLM neurons, and tested the effect of direct application of exogenous Ang II in the CVLM on nociceptive behaviors. Although AT(1)-immunoreactive neurons occurred in the CVLM, only 3% of AT(1)-positive neurons were found to project to the dorsal horn, using double-immunodetection of the retrograde tracer cholera toxin subunit B. In behavioral studies, administration of Ang II (100 pmol) in the CVLM gave rise to hyperalgesia in both the tail-flick and formalin tests. This hyperalgesia was significantly attenuated by local administration of the AT(1) antagonist losartan. The present study demonstrates that Ang II can act on AT(1) receptors in the CVLM to modulate nociception. The effect on spinal nociceptive processing is likely indirect, since few AT(1)-expressing CVLM neurons were found to project to the spinal cord. The renin-angiotensin system may also play a role in other supraspinal areas implicated in pain modulation.

From neuroanatomy to gene therapy: searching for new ways to manipulate the supraspinal endogenous pain modulatory system

The endogenous pain modulatory system is a complex network of brain areas that control nociceptive transmission at the spinal cord by inhibitory and facilitatory actions. The balance between these actions ensures effective modulation of acute pain, while during chronic pain the pronociceptive effects appear to prevail. The mechanisms underlying this imbalance were studied as to the role of two medullary components of the pain modulatory system: the dorsal reticular nucleus and the caudal ventrolateral medulla, which function primarily as pronociceptive and antinociceptive centres, respectively. Both areas are connected with the spinal dorsal horn by closed reciprocal loops. In the spino-dorsal reticular nucleus loop, the ascending branch is strongly inhibited by spinal GABAergic neurons, which may act as a buffering system of the dorsal reticular nucleus-centred amplifying effect. In the spino-caudal ventrolateral medulla loop, the ascending branch is under potent excitation of substance P (SP) released from primary afferents, which is likely to trigger the intense descending inhibition detected in acute pain. During chronic pain, the activity in the lateral reticular formation of the caudal ventrolateral medulla changes, so that the action of the caudal ventrolateral medulla upon SP-responsive spinal neurons shifts from inhibitory to excitatory. The mechanisms of this modulatory shift are unknown but probably relate to the decreased expression of micro-opioid, delta-opioid and GABAB receptors. Normalizing receptor expression in the caudal ventrolateral medulla or controlling noci-evoked activity at the dorsal reticular nucleus or caudal ventrolateral medulla by interfering with neurotransmitter release is now possible by the use of gene therapy, an approach that stands out as a unique tool to manipulate the supraspinal endogenous pain control system.

New GABAergic interneurons supported by myelin-specific T cells are formed in intact adult spinal cord

Neural stem/progenitor cells are known to exist in the intact spinal cord, but the presence of newly formed neurons during adulthood has not been documented there to date. Here, we report the appearance of newly formed neurons under normal physiological conditions. These neurons are immature, express a GABAergic phenotype, and are primarily located in the dorsal part of the spinal cord. This localization appeared to be mediated by stromal-derived factor-1/CXC-chemokine receptor-4 signaling in the dorsal region. The extent of spinal cord neurogenesis was found to be greatly influenced by immune system integrity and in particular by myelin-specific T cells. These observations provide evidence for in vivo spinal cord neurogenesis under nonpathological conditions and introduce novel mechanisms regulating adult spinal cord plasticity.