BACKGROUND
The peculiarities of pain perception in people of different ages should be considered in diagnosis and treatment of some pain syndromes. Pain is the most common symptom in elderly people and occurs in 62-73% of patients. The effect of age on pain perception is still unclear. Studies of pain sensitivity in elderly patients result contradictory data. According to some authors, pain thresholds are increased in elderly patients. Other researchers report reduced or intact pain thresholds. Data on nociceptive responses and pain behavior in animals throughout their life are also ambiguous.
OBJECTIVE
To study the peculiarities of pain sensitivity (Tail flick test) in male Wistar rats of different ages (3, 10 and 18 months) after a single intraperitoneal injection of pentylenetetrazole (PTZ) in sub-convulsive and convulsive doses.
MATERIAL AND METHODS
There were two series of experiments according to the same scheme on the same animals of different ages. In all series, PTZ was injected once intraperitoneally at a sub-convulsive dose of 15 mg/kg and convulsive dose of 60 mg/kg. Control animals of the same age were injected with saline solution under similar experimental conditions. Severity of convulsive reaction in response to PTZ administration was assessed according to the generally accepted scale and expressed in scores. Pain threshold (PT) was assessed using the Tail flick test (Ugo Basile, Italy). This device ensures a focused light beam to be applied to the animal’s tail. The time of pain reaction onset (latency period, LP) is estimated considering the animal’s tail abduction. PT was measured in seconds. PT was determined prior to PTZ administration, after 30 min, 24 hours and 6 days.
RESULTS
No age-dependent changes in PT were found in rats before PTZ injection. However, sub-convulsive and convulsive doses of PTZ had age-dependent features of PT. PTZ administration in 3-month-old animals at a sub-convulsive dose had a pronociceptive effect, injection of convulsive dose — antinociceptive effect. In animals aged 10 months, changes in pain sensitivity after PTZ administration were other. Sub-convulsive dose of PTZ did not affect the PT, administration of convulsive dose caused a short-term antinociceptive effect only in 30 minutes after injection. In the same animals aged 18 months, sub-convulsive and convulsive doses of PTZ caused antinociceptive effect throughout the entire follow-up period (6 days). In the control group, we found the peculiarities of age-related changes of PT after saline solution injection. These patterns were similar to those after injection of sub-convulsive dose of PTZ (reduced PT after saline solution injection at the age of 3 and 10 months and increased PT at the age of 18 months).
CONCLUSION
Administration of sub-convulsive doses of PTZ results a nociceptive effect in young animals and antinociceptive effect in aging animals. Injection of convulsive doses of PTZ has an antinociceptive effect regardless the age of the animals.
