Effects of epinephrine and cortisol on the analgesic activity of metyrosine in rats

Systemic administration of a β2-adrenergic receptor agonist reduces mechanical allodynia and suppresses the immune response to surgery in a rat model of persistent post-incisional hypersensitivity

Beta 2 adrenergic receptor (β2 AR) activation in the central and peripheral nervous system has been implicated in nociceptive processing in acute and chronic pain settings with anti-inflammatory and anti-allodynic effects of β2-AR mimetics reported in several pain states. In the current study, we examined the therapeutic efficacy of the β2-AR agonist clenbuterol in a rat model of persistent postsurgical hypersensitivity induced by disruption of descending noradrenergic signaling in rats with plantar incision. We used growth curve modeling of ipsilateral mechanical paw withdrawal thresholds following incision to examine effects of treatment on postoperative trajectories. Depletion of spinal noradrenergic neurons delayed recovery of hypersensitivity following incision evident as a flattened slope compared to non-depleted rats (-1.8 g/day with 95% CI -2.4 to -1.085, p < 0.0001). Chronic administration of clenbuterol reduced mechanical hypersensitivity evident as a greater initial intercept in noradrenergic depleted (6.2 g with 95% CI 1.6 to 10.8, p = 0.013) and non-depleted rats (5.4 g with 95% CI 1.2 to 9.6, p = 0.018) with plantar incision compared to vehicle treated rats. Despite a persistent reduction in mechanical hypersensitivity, clenbuterol did not alter the slope of recovery when modeled over several days (p = 0.053) or five weeks in depleted rats (p = 0.64). Systemic clenbuterol suppressed the enhanced microglial activation in depleted rats and reduced the density of macrophage at the site of incision. Direct spinal infusion of clenbuterol failed to reduce mechanical hypersensitivity in depleted rats with incision suggesting that beneficial effects of β2-AR stimulation in this model are largely peripherally mediated. Lastly, we examined β2-AR distribution in the spinal cord and skin using in-situ hybridization and IHC. These data add to our understanding of the role of β2-ARs in the nervous system on hypersensitivity after surgical incision and extend previously observed anti-inflammatory actions of β2-AR agonists to models of surgical injury.

The suppression of endogenous adrenalin in the prolongation of ketamine anesthesia

This study investigated whether or not the anesthetic effect of ketamine in rats is dependent on adrenal gland hormones. The study was performed on two main rat groups, intact and adrenalectomized. Rat were divided into subgroups and given appropriate doses of ketamine, metyrapone or metyrosine. Durations of anesthesia in the groups were then recorded. Endogenous catecholamine levels were measured in samples taken from peripheral blood. This experimental results showed that ketamine did not induce anesthesia in intact rats at doses of 15 or 30mg/kg, and that at 60mg/kg anesthesia was established for only 11min. However, ketamine induced significant anesthesia even at a dose of 30mg/kg in animals in which production of endogenous catecholamine (adrenalin, noradrenalin dopamine) was inhibited with metyrosine at a level of 45-47%. Ketamine at 60mg/kg in animals in which endogenous catecholamine was inhibited at a level of 45-47% established anesthesia for 47.6min. However, ketamine at 30 and 60mg/kg induced longer anesthesia in adrenalectomized rats with higher noradrenalin and dopamine levels but suppressed adrenalin production. Adrenalin plays an important role in the control of duration of ketamine anesthesia, while noradrenalin, dopamine and corticosterone have no such function. If endogenous adrenalin is suppressed, ketamine can even provide sufficient anesthesia at a 2-fold lower dose. This makes it possible for ketamine to be used in lengthy surgical procedures.

Suppression of adrenal gland-derived epinephrine enhances the corticosterone-induced antinociceptive effect in the mouse formalin test

BACKGROUND

There is both clinical and experimental evidence to support the application of corticosterone in the management of inflammation and pain. Corticosterone has been used to treat painful inflammatory diseases and can produce antinociceptive effects. Epinephrine is synthesized from norepinephrine by the enzyme phenylethanolamine N-methyltransferase (PNMT) and works as an endogenous adrenoceptor ligand secreted peripherally by the adrenal medulla. It is currently unclear whether corticosterone's antinociceptive effect is associated with the modulation of peripheral epinephrine.

METHODS

We first determined whether exogenous corticosterone treatment actually produced an antinociceptive effect in a formalin-induced pain model, and then examined whether this corticosterone-induced antinociceptive effect was altered by suppression of adrenal-derived epinephrine, using the following three suppression methods: (1) inhibition of the PNMT enzyme; (2) blocking peripheral epinephrine receptors; and (3) adrenalectomy.

RESULTS

Exogenous treatment with corticosterone at a high dose (50 mg/kg), but not at lower doses (5, 25 mg/kg), significantly reduced pain responses in the late phase. Moreover, injection of 2,3-dichloro-a-methylbenzylamine, a PNMT enzyme inhibitor, (10 mg/kg) before corticosterone treatment caused a leftward shift in the dose-response curve for corticosterone and injection of propranolol (5 mg/kg), but not phentolamine, also shifted the dose-response curve to the left during the late phase. Chemical sympathectomy with 6-hydroxydopamine had no effect on corticosterone-induced antinociceptive effect, but injection of a low dose of corticosterone produced an antinociceptive effect in adrenalectomized animals.

CONCLUSIONS

These results demonstrate that suppression of epinephrine, derived from adrenal gland, enhances the antinociceptive effect of exogenous corticosterone treatment in an inflammatory pain model.