Differential effect of dexamethasone on the antinociception induced by GABA receptor agonists or glutamate receptor antagonists administered spinallyin the mouse

It is not well identified that effect of dexamethasone in the antinocicepti on, when administered spinally as well as its interaction with GABAergic an d glutamatergic systems. We investigated to determine the possible role of glucocorticoid in the regulation of the anti-nociception induced by GABA re ceptor agonists and glutamate receptor antagonists treated spinally, using the tail-flick test. Mice were injected i.t. with a dose of dexamethasone ( 400 ng) in the presence or absence of muscimol (100 ng), baclofen (10 ng), MK-801(2 mug), or CNQX (1 mug). When dexamethasone was co-administered spin ally with muscimol or CNQX, the antinociceptive response was synergisticall y increased. Mice were pretreated with yohimbine (from 1 to 20 mug), methys ergide (from 1 to 20 mug), or naloxone (from 0.1 to 2 mug). And then, musci mol plus dexamethasone or CNQX plus dexamethasone was injected i.t. The pre treatment with yohimbine, methysergide, or naloxone attenuated the inhibiti on of the tail-flick response induced by muscimol plus dexamethasone or CNQ X plus dexamethasone in a dose-dependent manner. It is suggested that spina l alpha (2)-adrenergic and serotonergic receptors may be involved in spinal ly administered dexamethasone plus muscimol as well as dexamethasone plus C NQX. Also, it is speculated that opioids can be released by administration of dexamethasone plus muscimol or CNQX, and then stimulation of opioid rece ptors may be responsible for the synergistic antinociceptive interaction ob served between glucocorticoid and GABA,, or non-NMDA system.