Abnormal mitochondrial activity in pain-processing areas of the brain may explain why some persons with type 2 diabetes experience painful peripheral neuropathy while others do not, new U.K. study findings have suggested.

A greater ratio of adenosine triphosphate (ATP) – “the cellular energy currency of all life” – to phosphocreatine (PCr) was observed in the somatosensory cortex and right thalamus in those with painful diabetic peripheral neuropathy (DPN). Importantly, this correlated with neuropathic pain symptom intensity as measured by the Neuropathic Pain Symptom Inventory (NPSI) and the Doleur Neuroathique en 4 (DN4).

The findings suggest that altered cerebral phosphorus metabolite ratios may serve as a biomarker of DPN, said the study’s investigators.

“Normally the ATP:Cr ratio will be unaltered, but there’s stress to the brain that might change,” Gordon Sloan, a clinical research fellow within the Diabetes Research Unit at the Royal Hallamshire Hospital in Sheffield (England) said at the virtual annual meeting of the European Association for the Study of Diabetes.

DPN affects around a quarter of patients with type 2 diabetes but treatments are “inadequate”, and “unfortunately fewer than a third of individuals receive 50% or greater pain relief from current neuropathic pain treatments,” Mr. Sloan said. “Ultimately, this lack of understanding of the pathophysiology of the condition is therefore clear rationale to investigate the disease mechanisms further and to find novel targets for treatments,” he added.

Brain metabolites offer clues to neuropathic pain levels

The thalamus and primary somatosensory cortex are two key areas of the brain that are involved in the perception of painful stimuli, Mr. Sloan explained. “The thalamus receives most of the slowest sensory impulses from the peripheral nervous system modulating and processing them for relaying the signals to the rest of the pain matrix, including the somatosensory cortex where these sensations are interpreted and localized.”

Prior imaging work by Mr. Sloan’s group and others have shown that there are alterations in the functioning of both these brain areas in those with painful DPN versus healthy volunteers and those with type 2 diabetes but no DPN. So for their current study, Mr. Sloan and associates from Sheffield University and Sheffield Teaching Hospitals National Health Service Trust, used an advanced imaging method – phosphorus magnetic resonance spectroscopy (MRS) – to scan the thalamus and somatosensory cortex of 43 persons with type 2 diabetes and 12 healthy volunteers. Of those with diabetes, 11 had no DPN, 12 had DPN but were not currently in pain, and 20 had painful DPN.

From the scans, three phosphorus metabolite ratios were calculated, which gave an indication of mitochondrial activity: first, the ATP to PCr ratio, which gives a measure of cellular energy status; second, the ATP to inorganic phosphate (Pi) ratio, which measures oxidative phosphorylation; and third, the ratio of phosphomonoesters (PME) to phosphodiesters (PDE), which gives a measure of cell membrane turnover.

“We have measured the ratio of high-energy phosphate levels which are an indirect representation of the balance between energy generation, reserve and usage in the brain,” Mr. Sloan said.

The subjects studied were of a similar age, around 63 years on average, and well matched in terms of their sex and body mass index. Those with diabetes of course had higher blood glucose and glycated hemoglobin than did the healthy volunteers during the scans. Among those with diabetes, those with DPN were significantly more likely to have a longer duration of diabetes (12.5 years for painful DPN and 15.8 years for nonpainful DPN) than were those with no DPN (8.7 years).

Furthermore, those with DPN had higher scores on the Neuropathic Pain Symptom Inventory (NPSI) than did those without, although there was not much difference between those with painful or nonpainful DPN. On the other had, those with painful DPN were more likely to have higher scores when using the Doleur Neuroathique en 4 (DN4) to assess their pain level.

Results showed significant changes in cerebral cellular bioenergetics in the pain processing regions of the brain in those with painful DPN. The ATP:PCr at the thalamus and at the somatosensory cortex was significantly higher in those with painful DPN, compared with healthy volunteers. The other measures of phosphorus metabolite levels (ATP:Pi and PME:PDE) were unaltered.

“We hypothesize that the findings of the study are suggestive of increased energy demands in regions of pain perception due to increased neuronal activity” said Dr. Sloan.

The study’s results add further evidence for cerebral alterations playing a key role in the generation and maintenance of pain in painful DPN.

SOURCE: Sloan S et al. EASD 2020, oral presentation 181.

Diabetic neuropathic pain linked to brain bioenergic anomalies