Research yields new insights into mechanisms and treatment of pain

Differential roles of hippocampal glutamatergic receptors in neuropathic anxiety-like behavior after partial sciatic nerve ligation in rats

BACKGROUND

Neuropathic pain evoked by nerve injury is frequently accompanied by deterioration of emotional behaviors, but the underlying signaling mechanisms remain elusive. Glutamate (Glu) is the major mediator of excitatory synaptic transmission throughout the brain, and abnormal activity of the glutamatergic system has been implicated in the pathophysiology of pain and associated emotional comorbidities. In this study we used the partial sciatic nerve ligation (PSNL) model of neuropathic pain in rats to characterize the development of anxiety-like behavior, the expression of glutamatergic receptors, and the phosphorylation of extracellular signal-regulated kinase (ERK) in the hippocampus, the region that encodes memories related to emotions.

RESULTS

We found that the mechanical withdrawal threshold was significantly reduced and an anxiety-like behavior was increased as determined via open field tests and elevated plus-maze tests at 28 days after injury. No significant differences were found in the ratio of sucrose preference and immobility time detected by sucrose preference tests and forced swimming tests respectively, possibly due to the timing factor. The expression of N-methyl-D-aspartate (NMDA) receptor subtypes NR1 and NR2B, but not NR2A, GluR1, or GluR2 (the main subtype of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] receptor) in the hippocampus of injured rats was significantly reduced. Moreover, PSNL resulted in decreased phosphorylation of ERK1/2 in the hippocampus. Intriguingly, treatment with D-serine (a co-agonist of NMDA receptor, 1 g/kg intraperitoneally) reduced the anxiety-like behavior but not the mechanical hypersensitivity induced by PSNL.

CONCLUSIONS

PSNL can induce significant anxiety-like but not depression-like behavior, and trigger down-regulation of NMDA but not AMPA receptors in the hippocampus at 28 days after injury.

Newly initiated opioid treatment and the risk of fall-related injuries. A nationwide, register-based, case-crossover study in Sweden

BACKGROUND

There is growing epidemiological evidence that opioids may increase the risk of unintentional injuries and it is plausible that the time of initiation is most critical in that respect. Studies on fall-related injuries remain few, limited and mostly focused on specific groups of elderly patients.

OBJECTIVE

The objective of this study was to assess the short-term effects of newly prescribed opioids on the risk of fall-related injuries in the general adult population.

METHODS

A case-crossover design was applied on national register data linking, at the individual level, fall-injury information involving adults aged 18 years and above identified in the Swedish National Inpatient Register (during the period 1 May 2006 to 31 December 2009) and dispensed drugs from the Swedish Prescribed Drug Register (n=167,257 cases with a first fall-related injury). All types of opioid substances were considered, classified according to the Anatomical Therapeutic Chemical (ATC) classification system. We investigated newly dispensed opioids 28 days preceding the injury, compared with an earlier, and equally long, control period following a 3-month washout period. Conditional logistic regression was used to estimate odds ratio (OR) and 95% confidence interval (CI). The analyses were also conducted stratified by age group, by type of fall and for each period of 1 week during the 28-day period.

RESULTS

From among the fall-injured patients, 7,450 patients (4.5%) had a new opioid dispensation within 28 days prior to the injury, of which the most frequent types were tramadol (2.0%) and codeine (1.1%). Consistently increased risks of fall-related injuries associated with a new prescription of any opioid were found and they were most pronounced among young adults, 18-29 years of age (OR, 7.17; 95% CI 5.04-10.2). The closer the dispensation date to the injury, the higher the odds: an OR of 5.14 (95% CI 4.76-5.55) during the first week of opioid treatment and 1.23 (95% CI 1.10-1.38) for the fourth week. Of the documented falls, the risk was most pronounced for falls from 'another, high level' (OR, 5.33; 95% CI 3.99-7.10).

CONCLUSIONS

Newly prescribed opioids may trigger injurious falls. The effect lowers over time and is less pronounced with increasing age. The risk is also higher for fall from height.