Infra-slow oscillations in chronic orofacial neuropathic pain and the effects of palmitoylethanolamide

For centuries, chronic pain was denied as being real by physicians, mainly because there was no evidence of tissue damage. The lack of understanding of the neural mechanisms underlying chronic pain, particular that arising from nervous system damage, has hindered treatment development which has led to the over-prescription of opioids. Whilst the brain circuitry responsible for the perception of acute painful stimuli have been mapped in both animal studies and studies using brain imaging in awake humans, the circuitry responsible for the initiation and maintenance of chronic pain remain unknown. Over the past few decades, many human brain imaging investigations have shown that neuropathic pain is associated with altered brain rhythms and in particular thalamocortical dysrhythmia. In addition, animal studies have shown that neuropathic pain is associated with altered non-neural function including microglial and astrocyte activation at the level of the primary afferent synapse. These results have led to theories that non-neuronal cells may be crucial for the initiation and maintenance of chronic pain, particularly chronic neuropathic pain. It has been a long held view that astrocytes mainly play the role of neural support in the central nervous system, however, these cells are also capable of controlling neural function. In fact, astrocytes have access to every neural synapse and animal models of chronic neuropathic pain have shown that targeting astrocytes can control pain intensity. As such, the focus of this thesis is to identify the role of astrocytes in modulating neural function in chronic neuropathic pain and to determine whether reducing astrocyte activity can reduce pain intensity. There are three main investigations that make up this thesis, the first describes an experimental procedure whereby on-going patterns of neural activity were assessed in patients with orofacial neuropathic pain using resting state functional magnetic resonance imaging. The second attempts to measure an anatomical marker of astrocyte activation. And the final investigation describes an experimental procedure whereby patients with orofacial neuropathic pain were administered an astrocyte modulator, palmitoylethanolamide (PEA) and neural activity was compared before and after treatment

Effects of the glial modulator palmitoylethanolamide on chronic pain intensity and brain function.

Background: Chronic neuropathic pain (NP) is a complex disease that results from damage or presumed damage to the somatosensory nervous system. Current treatment regimens are often ineffective. The major impediment in developing effective treatments is our limited understanding of the underlying mechanisms. Preclinical evidence suggests that glial changes are crucial for the development of NP and a recent study reported oscillatory activity differences within the ascending pain pathway at frequencies similar to that of cyclic gliotransmission in NP. Furthermore, there is evidence that glial modifying medications may be effective in treating NP. The aim of this Phase I open-label clinical trial is to determine whether glial modifying medication palmitoylethanolamide (PEA) will reduce NP and whether this is associated with reductions in oscillatory activity within the pain pathway. Methods: We investigated whether 6 weeks of PEA treatment would reduce pain and infra-slow oscillatory activity within the ascending trigeminal pathway in 22 individuals (17 females) with chronic orofacial NP. Results: PEA reduced pain in 16 (73%) of the 22 subjects, 11 subjects showed pain reduction of over 20%. Whilst both the responders and non-responders showed reductions in infra-slow oscillatory activity where orofacial nociceptor afferents terminate in the brainstem, only responders displayed reductions in the thalamus. Furthermore, functional connections between the brainstem and thalamus were altered only in responders. Conclusion: PEA is effective at relieving NP. This reduction is coupled to a reduction in resting oscillations along the ascending pain pathway that are likely driven by rhythmic astrocytic gliotransmission

Altered Brainstem Pain-Modulation Circuitry Connectivity During Spontaneous Pain Intensity Fluctuations.

BackgroundChronic pain, particularly that following nerve injury, can occur in the absence of external stimuli. Although the ongoing pain is sometimes continuous, in many individuals the intensity of their pain fluctuates. Experimental animal studies have shown that the brainstem contains circuits that modulate nociceptive information at the primary afferent synapse and these circuits are involved in maintaining ongoing continuous neuropathic pain. However, it remains unknown if these circuits are involved in regulating fluctuations of ongoing neuropathic pain in humans.MethodsWe used functional magnetic resonance imaging to determine whether in 19 subjects with painful trigeminal neuropathy, brainstem pain-modulation circuitry function changes according to moment-to-moment fluctuations in spontaneous pain intensity as rated online over a 12-minute period.ResultsWe found that when pain intensity was spontaneously high, connectivity strengths between regions of the brainstem endogenous pain-modulating circuitry-the midbrain periaqueductal gray, rostral ventromedial medulla (RVM), and the spinal trigeminal nucleus (SpV)-were high, and vice-versa (when pain was low, connectivity was low). Additionally, sliding-window connectivity analysis using 50-second windows revealed a significant positive relationship between ongoing pain intensity and RVM-SpV connectivity over the duration of the 12-minute scan.ConclusionThese data reveal that moment-to-moment changes in brainstem pain-modulation circuitry functioning likely contribute to fluctuations in spontaneous pain intensity in individuals with chronic neuropathic pain

Effects of the glial modulator palmitoylethanolamide on chronic pain intensity and brain function.

Background: Chronic neuropathic pain (NP) is a complex disease that results from damage or presumed damage to the somatosensory nervous system. Current treatment regimens are often ineffective. The major impediment in developing effective treatments is our limited understanding of the underlying mechanisms. Preclinical evidence suggests that glial changes are crucial for the development of NP and a recent study reported oscillatory activity differences within the ascending pain pathway at frequencies similar to that of cyclic gliotransmission in NP. Furthermore, there is evidence that glial modifying medications may be effective in treating NP. The aim of this Phase I open-label clinical trial is to determine whether glial modifying medication palmitoylethanolamide (PEA) will reduce NP and whether this is associated with reductions in oscillatory activity within the pain pathway. Methods: We investigated whether 6 weeks of PEA treatment would reduce pain and infra-slow oscillatory activity within the ascending trigeminal pathway in 22 individuals (17 females) with chronic orofacial NP. Results: PEA reduced pain in 16 (73%) of the 22 subjects, 11 subjects showed pain reduction of over 20%. Whilst both the responders and non-responders showed reductions in infra-slow oscillatory activity where orofacial nociceptor afferents terminate in the brainstem, only responders displayed reductions in the thalamus. Furthermore, functional connections between the brainstem and thalamus were altered only in responders. Conclusion: PEA is effective at relieving NP. This reduction is coupled to a reduction in resting oscillations along the ascending pain pathway that are likely driven by rhythmic astrocytic gliotransmission