ADRENALECTOMY ENHANCES FOS-LIKE IMMUNOREACTIVITY WITHIN THE SPINAL TRIGEMINAL NUCLEUS INDUCED BY NOXIOUS THERMAL-STIMULATION OF THE CORNEA

Immunocytochemistry was used to assess the distribution of neurons wit hin the spinal trigeminal nucleus that expressed the protein product o f the proto-oncogene c-fos after thermal stimulation of the cornea in barbiturate-anesthetized rats. The influence of adrenal steroids on Fo s-like immunoreactivity induced by corneal stimulation also was examin ed by comparison of the results obtained in adrenal intact rats to tho se in adrenalectomized rats and to those in adrenalectomized rats give n corticosterone replacement therapy. Stimuli (42 or 52-degrees-C, 20 s per min, 15 min) were applied unilaterally to the cornea by a contac t thermode. At 2 h after stimulation animals were perfused with 4% par aformaldehyde and tissue sections were incubated with primary antiseru m against the Fos protein and processed with the avidin-biotin method. The pattern of Fos-like immunoreactivity after 52-degrees-C stimulati on revealed a dominant group of cells ipsilaterally within the superfi cial laminae of the caudalmost portion of trigeminal subnucleus caudal is that was greatly enhanced in adrenalectomized rats. Low-intensity s timulation did not induce Fos-like immunoreactivity among cells in thi s caudal region. A second significant group of cells was seen more ros trally at periobex levels within the ventrolateral pole of the nucleus . The number of cells in the periobex grouping was increased after 52 or 42-degrees-C corneal stimulation when compared to unstimulated cont rols and was not affected by levels of corticosterone. The results ind icated a discontinuous pattern of Fos-like immunoreactivity within the spinal trigeminal nucleus after thermal stimulation of the cornea and a differential effect of adrenal steroids. The appearance of Fos-like immunoreactivity within caudal portions of the nucleus was increased only by noxious intensities of stimulation and was further enhanced in animals with low levels of corticosterone. In contrast, the Fos-like immunoreactivity at periobex levels was increased after noxious and in nocuous thermal stimuli and was independent of the level of corticoste rone. The results were consistent with the hypothesis that glucocortic oids modify the expression of immediate early genes among a select gro up of central trigeminal neurons. Such steroid modulation may contribu te to the mechanisms that underlie long-term adaptation to noxious sen sory input.