C-Fos expression in trigeminal nucleus neurons after chemical irritation of the cornea: reduction by selective blockade of nociceptor chemosensitivity.

The distribution and number of trigeminal brainstem and higher order sensory neurons expressing the protein product of the proto-oncogene c-fos after noxious stimulation of the cornea was studied in the rat using immunocytochemistry. The possibility that attenuation of nociceptive messages from the cornea by diltiazem reduced Fos-like immunoreactivity of spinal trigeminal neurons was also examined. A group of animals were killed 2-3 h after corneal stimulation. One cornea was stimulated with: a drop of 10 mM acetic acid; with acid plus mechanical scratching of the corneal epithelium; or with a drop of saline of 56 degrees C. Half of the animals treated with acid had been pretreated ipsilaterally with topical diltiazem (10 mM). Control rats received either saline in one eye or no treatment. Another group of animals were killed 7-8 h after stimulation with acetic acid. Fos-like immunoreactive neurons were counted in serial brainstem sections using an anti-Fos primary antiserum and processed according to the avidin-biotin complex method. In rats killed 2-3 h after corneal stimulation with acid, heat, or acid plus mechanical injury, labelled neurons were found in laminae I and II of the intermediate zone between caudalis and interpolaris subnuclei of the ipsilateral spinal trigeminal nucleus and, in a reduced number, in the symmetrical zones of the contralateral side. In animals stimulated with noxious heat or combined mechanical and chemical injury, a few scattered cells were also labelled in the ipsilateral junction between the cervical spinal cord and the caudalmost part of the trigeminal subnucleus caudalis. In rats killed 7 h after stimulation with acid, stained neurons were observed in the same areas of the trigeminal nucleus as in rats killed at shorter times, but in lower numbers; in these animals, no immunoreactive cells were found in deeper laminae or in higher sensory relay nuclei. Pretreatment with diltiazem significantly reduced the number of cells of the spinal trigeminal nucleus labelled after corneal stimulation with acid. The results indicate that brief noxious stimulation of the cornea evoke expression of c-Fos in neurons of the spinal trigeminal complex. Diminution by diltiazem of the number of immunoreactive neurons activated by corneal irritation suggests that this drug, by reducing chemosensitivity of nociceptive terminals, decreases nociceptive inflow to central nervous structures involved in ocular pain perception.