ACUTE INJECTION OF ADRENAL-STEROIDS REDUCES CORNEA-EVOKED EXPRESSION OF C-FOS WITHIN THE SPINAL TRIGEMINAL NUCLEUS OF ADRENALECTOMIZED RATS

The influence of transient increases in adrenal steroid hormones on th e number of Fos-positive neurons after nociceptor activation was asses sed in adrenalectomized rats. Fos protein, the product of the immediat e early gene, c-fos, was detected immunocytochemically within the spin al trigeminal nucleus 2 h after noxious thermal stimulation of the cor nea. Adrenalectomized rats displayed an enhanced number of Fos-positiv e neurons within the caudal-most portions of trigeminal subnucleus cau dalis compared to that seen in adrenal-intact animals, an effect rever sed by a single acute injection of corticosterone (1 mg/kg, i.p.) give n 5 min prior to stimulation. Acute injection of the selective mineral ocorticoid receptor agonist, aldosterone, or the selective glucocortic oid receptor agonist, RU28362, also reduced the number of Fos-positive neurons. Aldosterone and RU28362 had an additive effect on Fos when g iven concurrently. In contrast, adrenal status or acute injections of adrenal steroid receptor agonists had no effect on the number of Fos-p ositive neurons after corneal stimulation located within the ventrolat eral pole of the spinal trigeminal nucleus at the level of the subnucl eus interpolaris/caudalis junction. Acute administration of adrenal st eroids to adrenalectomized rats greatly attenuated the number of Fos-p ositive neurons seen after corneal stimulation within select portions of trigeminal subnucleus caudalis. The contribution of both glucocorti coid and mineralocorticoid receptor subtypes in reducing Fos suggested a central site of action rather than an anti-inflammatory effect on p eripheral tissue. These results are consistent with the hypothesis tha t transient increases in adrenal steroids, such as occur after injury, are sufficient to modify the production of Fos protein in central neu rons that process nociceptive information.