N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonism reduces Fos-like immunoreactivity in central trigeminal neurons after corneal stimulation in the rat

The role of glutamate receptors in processing noxious sensory input from the cornea was assessed in barbiturate-anesthetized rats. Animals were treated with selective antagonists for N-methyl-D-aspartate or non-N-methyl- D-aspartate receptor subtypes prior to application of mustard oil to the corneal surface. Neural activation was estimated from the number of neurons that produced Fos, the protein product of the immediate early gene, c-fos, as detected by immunocytochemistry. Fos-positive neurons were found at two distinct regions of the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical cord transition. The number of Fos-positive neurons was reduced dose- dependently by the competitive N-methyl-D-aspartate receptor antagonist, 3- [(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (0.08-8 nmol, i.c.v.), or by the non-N-methyl-D-aspartate receptor antagonist, 6,7- dinitroquinoxaline-2,3-dione (2.5-250 nmol, i.c.v.). The greatest reduction in Fos-positive cells was seen at the subnucleus caudalis/upper cervical cord transition after blockade of either receptor subtype. Combined blockade of N- methyl-D-aspartate and non-N-methyl-D-aspartate receptors did not cause a further reduction in the number of Fos-positive neurons than was seen after the highest dose of either antagonist alone. Peripheral or central administration of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester, had no effect on the number of Fos-positive neurons after corneal stimulation. These results suggest that corneal input to neurons at the subnucleus caudalis/upper cervical cord transition, and to a lesser extent, at the subnucleus interpolaris/subnucleus caudalis transition depends on excitatory amino acid transmission. Both N-methyl-D-aspartate and non-N- methyl-D-aspartate glutamate receptor subtypes, but not the formation of nitric oxide, contribute to the processing of acute corneal stimuli by central trigeminal neurons.