GABA(A) receptor-mediated effects on expression of c-Fos in rat trigeminalnucleus following high- and low-intensity afferent stimulation

We examined the effects of systemic administration of a GABA(A) receptor ag onist, muscimol, or antagonist, bicuculline, on the expression of c-Fos pro tein induced 3 h after electrical stimulation of the trigeminal ganglion at low (0.1 mA) or high intensities (1.0 mA) in the urethane-anesthetized rat . In saline-treated rats, 10 min stimulation of the trigeminal ganglion ind uced c-Fos-immunopositive neurons throughout the full extent of the ipsilat eral superficial layers of the trigeminal nucleus caudalis, and dorsal or d orsomedial Dart of the nuclei rostral to obex (trigeminal nucleus principal is, dorsomedial nucleus of trigeminal nucleus oralis, dorsomedial nucleus o f trigeminal nucleus interpolaris). Animals stimulated at 1.0 mA induced a significantly higher number of labeled neurons in all trigeminal sensory nu cleus than animals stimulated at 0.1 mA. In rats treated with 1 mg/kg i.p. muscimol and stimulated at 0.1 mA, the numbers of Fos-positive neurons in t rigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus inter polaris, and dorsomedial nucleus of trigeminal nucleus oralis were signific antly decreased. However, after stimulation at 1.0 mA, the numbers of Fos-p ositive neurons in the superficial layers of trigeminal nucleus caudalis wa s increased and no changes occurred in the numbers of Fos-positive neurons in the magnocellular zone of trigeminal nucleus caudalis, the dorsomedial n ucleus of trigeminal nucleus interpolaris, or dorsomedial nucleus of trigem inal nucleus oralis compared to saline-treated controls. In rats treated wi th 2 mg/kg i.p. bicuculline and stimulated at 0.1 mA, the number of Fos-pos itive neurons increased in the superficial layers of trigeminal nucleus cau dalis and trigeminal nucleus principalis. However, after stimulation at 1.0 mA, the number of Fos-positive neurons was unchanged in superficial layers of trigeminal nucleus caudalis, but decreased in the magnocellular zone of trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus int erpolaris and dorsomedial nucleus of trigeminal nucleus oralis. There was a specific loss of Fos-positive neurons in the maxillary and ophthalmic divi sions (ventrolateral half) of trigeminal nucleus caudalis. These results indicate that the expression of c-Fos in the trigeminal nucle us is differentially regulated through GABA(A) receptors in a manner that i s dependent on the nucleus and the type of primary afferents that are activ ated by different stimulus intensities. (C) 2000 IBRO. Published by Elsevie r Science Ltd. All rights reserved.