Caudal portions of the spinal trigeminal complex are necessary for autonomic responses and display Fos-like immunoreactivity after corneal stimulation in the cat

Corneal input to the spinal trigeminal nucleus (Vsp) was assessed by examining Fos-like immunoreactivity (Fos-LI) after chemical irritant stimulation by mustard oil in chloralose-anesthetized cats. The distribution of Fos-LI within the ipsilateral Vsp was bimodal: a dominant group of cells within the superficial laminae at caudal levels of subnucleus caudalis and a second group of cells within the ventrolateral pole of Vsp at obex levels and within the adjacent interstitial islands. Few Fos-positive cells were seen within the Vsp rostral to the mid-portion of subnucleus interpolaris or within the contralateral Vsp. To assess the involvement of caudal portions of the Vsp in mediating the adrenal and autonomic responses to corneal stimulation, mustard oil was applied before and after lidocaine blockade of the Vsp at obex levels in a second group of cats. Corneal stimulation alone increased significantly (P < 0.001) the adrenal secretion of catecholamines, adrenal blood flow, mean arterial pressure and heart rate. With the exception of heart rate, the adrenal and autonomic responses to mustard oil were greatly attenuated or abolished by lidocaine blockade of the ipsilateral Vsp at the level of the obex, a region that displayed a high number of Fos-positive cells after corneal stimulation. These results indicate that neurons within the Vsp at or more caudal than the level of the obex process chemical irritant input from the cornea and are necessary for corneal-evoked changes in adrenal and autonomic function.