We examined the effects of systemic administration of a GABA(A) receptor agonist, muscimol, or antagonist, bicuculline, on the expression of c-Fos protein induced 3h 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 induced c-Fos-immunopositive neurons throughout the full extent of the ipsilateral superficial layers of the trigeminal nucleus caudalis, and dorsal or dorsomedial part of the nuclei rostral to obex (trigeminal nucleus principalis, dorsomedial nucleus of trigeminal nucleus oralis, dorsomedial nucleus of trigeminal nucleus interpolaris). Animals stimulated at 1. 0 mA induced a significantly higher number of labeled neurons in all trigeminal sensory nucleus than animals stimulated at 0.1 mA. In rats treated with 1mg/kg i.p. muscimol and stimulated at 0.1 mA, the numbers of Fos-positive neurons in trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus interpolaris, and dorsomedial nucleus of trigeminal nucleus oralis were significantly decreased. However, after stimulation at 1.0 mA, the numbers of Fos-positive neurons in the superficial layers of trigeminal nucleus caudalis was increased and no changes occurred in the numbers of Fos-positive neurons in the magnocellular zone of trigeminal nucleus caudalis, the dorsomedial nucleus of trigeminal nucleus interpolaris, or dorsomedial nucleus of trigeminal nucleus oralis compared to saline-treated controls. In rats treated with 2mg/kg i.p. bicuculline and stimulated at 0.1 mA, the number of Fos-positive neurons increased in the superficial layers of trigeminal nucleus caudalis 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 interpolaris and dorsomedial nucleus of trigeminal nucleus oralis. There was a specific loss of Fos-positive neurons in the maxillary and ophthalmic divisions (ventrolateral half) of trigeminal nucleus caudalis. These results indicate that the expression of c-Fos in the trigeminal nucleus is differentially regulated through GABA(A) receptors in a manner that is dependent on the nucleus and the type of primary afferents that are activated by different stimulus intensities.
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