The impact of age on emotional and cognitive behaviours triggered by experimental neuropathy in rats

Chronic pain syndromes encompass several clinical entities that frequently affect the individuals' emotional and cognitive behaviours which, in turn, can also alter pain perception. Additionally, both pain perception and motivational-affective behaviours change with increasing age. In order to evaluate the influence of age upon the interaction between chronic pain and affective/cognitive behaviours, 3-, 10- and 22-month-old rats with 1 month neuropathy (spared nerve injury, SNI model) were compared with age-matched sham-operated controls in the open field (OF; locomotor and exploratory behaviours), elevated plus-maze (EPM; anxiety-like behaviour), forced swimming (FST; depressive-like behaviour), working memory water maze (WM; spatial short-term memory), Morris water maze (MWM; spatial reference memory) and spatial reversal (behavioural flexibility) tests. Locomotor and exploratory activities decreased steadily with age and were further reduced by SNI. Aging was associated with increased anxiety-like behaviour, which was potentiated by SNI in both 3- and 22-month-old rats. The performance in the FST was affected by SNI but only in mid-aged animals. Cognitive performances in the MWM and spatial reversal tests deteriorated with age; however, the SNI lesion was only detrimental in the reversal task to mid-aged animals. Our data demonstrate that the influence of neuropathic pain on affective and cognitive behaviours is age dependent and varies with the behavioural domain that is tested. Importantly, mid-aged animals seem to be more susceptible to depression and cognitive deterioration associated to chronic pain than young and old groups.