Abstract
Background: Diffuse noxious inhibitory control
(DNIC) is thought to be mediated by neural
networks in supraspinal brain structures. The
descending antinociceptive system (DAS) is an
important component of the DNIC neural network,
but the precise structure of the neural network
and the related neurotransmitters have not been
examined.
Methods: The study was designed to examine
whether systemic administration of the
adrenergic agonists dexmedetomidine (DEX)
and phenylephrine (PE) influences DNIC in
the rat. Changes in the C-fiber reflex evoked
by electromyographic activity were recorded
following noxious tail immersion in hot water.
Results: Inhibition of the C-fiber reflex by the
conditioning stimuli was reduced from 77.1
± 22.6% to 26.6 ± 38.2% with continuous
administration of DEX, and restored to 58.3 ±
29.2% by intramuscular injection of atipamezole
hydrochloride(APZ), a selective α 2- adrenoceptor
antagonist. Inhibition of the C-fiber reflex was
reduced from 75.6 ± 25.8% to 22.7 ± 38.9% with
continuous administration of PE, and restored
to 84.9 ± 9.7% by intramuscular injection of
phentolamine mesylate (PT), an α - adrenoceptor
antagonist.
Conclusion: The results show that clinical doses
of DEX and PE inhibit DNIC, thereby aff ecting and
modulating the intrinsic pain inhibition system.
These findings suggest that adrenergic neurons
are involved in DNIC.
