A lack of supersensitivity to opioid receptor agonists following chronic spinal opioid receptor antagonist administration in the rat.

1. Male Sprague-Dawley rats were chronically tested with intrathecal (i.t.

Age-related changes in the spinal antinociceptive effects of DAGO, DPDPE and beta-endorphin in the rat.

These studies were designed to investigate how the aging process alters the spinal antinociceptive efficacy of mu (mu), delta (delta) and epsilon (epsilon) opioid receptor agonists administered intrathecally (i.t.) in rats.

Effects of aging on spinal opioid-induced antinociception.

Initial experiments were conducted to determine whether or not the aging process alters the ability of young, mature, or aged male Fischer 344 rats (5- to 6-, 15- to 16-, and 25- to 26-months-old, respectively) to respond to thermal nociceptive stimuli. Using the tail-flick analgesiometric assay, 25- to 26-month-old rats responded significantly faster to the heat source than 15- to 16-month-old animals, but no significant differences were noted between the 5- to 6-month-old and aged rats. Another series of investigations compared the effects of aging on the spinal antinociceptive properties of the mu opioid agonist [D-Ala2,N-methyl-Phe4,Gly5-ol] enkephalin (DAMPGO) and the delta agonist [D-Pen2,D-Pen5] enkephalin (DPDPE).

The noradrenergic component contributing to spinal fentanyl-induced antinociception is supraspinally mediated.

1. Male Sprague-Dawley rats were fitted with intrathecal (i.t.

A single restraint stress exposure potentiates analgesia induced by intrathecally administered DAGO.

In rats, restraint exposure potentiates the magnitude and duration of analgesia following both the peripheral and intracerebroventricular administration of several opioid agonists as compared to non-stressed controls. It has been suggested that the site of action whereby restraint leads to potentiated opioid analgesia is located supraspinally. However, the possible contribution of spinal analgesic mechanisms also warrants investigation.