Dorsal horn antinociception mediated by the paraventricular hypothalamic nucleus and locus coeruleous: A comparative study

The role of the endocannabinoid 2-arachidonoylglycerol in the in vivo spinal oxytocin-induced antinociception in male rats

Oxytocin receptor (OTR) activation at the spinal level produces antinociception. Some data suggest that central OTR activation enhances social interaction via an increase of endocannabinoids (eCB), but we do not know if this could occur at the spinal level, modulating pain transmission. Considering that oxytocin via OTR stimulates diacylglycerol formation, a key intermediate in synthesizing 2-arachidonylglycerol (2-AG), an eCB molecule, we sought to test the role of the eCB system on the spinal oxytocin-induced antinociception. Behavioral and electrophysiological experiments were conducted in naïve and formalin-treated (to induce long-term mechanical hypersensitivity) male Wistar rats. Intrathecal RHC 80267 injections, an inhibitor of the enzyme diacylglycerol lipase (thus, decreasing 2-AG formation), produces transient mechanical hypersensitivity, an effect unaltered by oxytocin but reversed by gabapentin. Similarly, in in vivo extracellular recordings of naïve spinal wide dynamic range cells, juxtacellular picoinjection of RHC 80267 increases the firing of nociceptive Aδ-, C-fibers, and post-discharge, an effect unaltered by oxytocin. Interestingly, in sensitized rats, oxytocin picoinjection reverses the RHC 80627-induced hyperactivity of Aδ-fibers (but not C- or post-discharge activity). In contrast, a sub-effective dose of JZL184 (a monoacylglycerol lipase inhibitor, thus favoring 2-AG levels), which does not have per se an antinociceptive effect in the formalin-induced hypernociception, the oxytocin-induced antinociception is boosted. Similarly, electrophysiological experiments suggest that juxtacellular JZL184 diminishes the neuronal firing of nociceptive fibers, and co-injection with oxytocin prolongs and enhances the antinociceptive effect. These data may imply that 2-AG formation may play a role in the spinal antinociception induced by oxytocin.

Translational aspects of deep brain stimulation for chronic pain

The use of deep brain stimulation (DBS) for the treatment of chronic pain was one of the first applications of this technique in functional neurosurgery. Established brain targets in the clinic include the periaqueductal (PAG)/periventricular gray matter (PVG) and sensory thalamic nuclei. More recently, the anterior cingulum (ACC) and the ventral striatum/anterior limb of the internal capsule (VS/ALIC) have been investigated for the treatment of emotional components of pain. In the clinic, most studies showed a response in 20%-70% of patients. In various applications of DBS, animal models either provided the rationale for the development of clinical trials or were utilized as a tool to study potential mechanisms of stimulation responses. Despite the complex nature of pain and the fact that animal models cannot reliably reflect the subjective nature of this condition, multiple preparations have emerged over the years. Overall, DBS was shown to produce an antinociceptive effect in rodents when delivered to targets known to induce analgesic effects in humans, suggesting a good predictive validity. Compared to the relatively high number of clinical trials in the field, however, the number of animal studies has been somewhat limited. Additional investigation using modern neuroscience techniques could unravel the mechanisms and neurocircuitry involved in the analgesic effects of DBS and help to optimize this therapy.

Role of central oxytocin and dopamine systems in nociception and their possible interactions: suggested hypotheses

Central oxytocin and dopamine have an important role in the process of nociception at the spinal level as well as supraspinal structures, e.g. anterior cingulate cortex, insular cortex, amygdala, nucleus accumbens, and hypothalamus. Many studies have pointed out the importance of both systems in the pain descending modulatory system and in pain-related symptoms in some chronic disorders, e.g. Parkinson disease and fibromyalgia. The interaction between oxytocin and dopamine systems has been addressed in some motivational behaviors, e.g. maternal and sexual behaviors, pair bonding, and salience. In this aspect, we propose that an oxytocin-dopamine interaction could be present in nociception, and we also explain the possible hypotheses of such an interaction between these systems.