[The influence of the thymus peptides on analgesia caused by acute and chronic immobilization]

Our aim was to investigate the influence of thymic polypeptides on pain sensitivity and to analyze a possible role of the opioid system in the implementation of the analgesia caused by immobilization stress.

The study was performed on male Wistar rats at the Moscow state University named after M. V. Lomonosov. We studied effects of thymus peptides: thymuline (0.15 mg/kg), fraction 5 thymosin (0.25 microgram/kg) and cattle thymus extracted product (CTEP) (0.5 mg/kg) on pain sensitivity in rats using test "tail flick" without stress, with acute (3 h) and sub acute (12 h) immobilization stress. The comparison groups were animals treated with saline and spleen polypeptides.

It is shown that preparations of thymus increase the threshold of pain sensitivity in the intact animals. Immobilization stress duration 3 and 12 h in thymus peptides treated rats caused a less pronounced increase in pain threshold than in the control groups (immobilization stress 3 h: CTEP--p = 0.025, thymuline--p = 0.022, fraction 5 thymosin--p = 0.033; immobilization stress 12 h: CTEP--p = 0.034, thymuline--p = 0.027, fraction 5 thymosin--p = 0.036). The opioid receptor blocker naloxone (1 mg/kg) did not completely block the stress-induced analgesia, indicating the presence of both opioid and non -opioid components in this state. In thymus peptides treated rats, opioid component was less pronounced than in the control groups (CTEP--p = 0.031, thymuline--p = 0.026, fraction 5 thymosin--p = 0.029).

Pre-activation of the opioid system by the thymus polypeptides leads to an increase in the share of non-opioid component of the stress-induced analgesia and prevents the depletion of the opioid system in immobilization stress.