NADPH-DIAPHORASE HISTOCHEMISTRY PROVIDES EVIDENCE FOR A BILATERAL, SOMATOTOPICALLY INAPPROPRIATE RESPONSE TO UNILATERAL HINDPAW INFLAMMATION IN THE RAT

Unilateral hindpaw inflammation produces several neurochemical changes in the ipsilateral spinal dorsal horn that have been interpreted as c ontributing to the associated hyperalgesia. NADPH-diaphorase histochem ical staining was used to examine the response of a population of neur ons 1, 2, 6 or 24 h following injection of 6 mg carrageenan into the l eft hindpaw of the rat. The resulting unilateral hindpaw inflammation produced a bilateral, time-dependent, reversible increase in the numbe r of NADPH-diaphorase stained neurons in the lumbar spinal cord that p eaked at 6 h. In laminae I-III, there was a significant increase in th e number of NADPH-diaphorase stained neurons both ipsilateral (25.9 +/ - 2.3) and contralateral (26.3 +/- 1.3) to the inflamed hindpaw relati ve to uninflamed, control animals (18.6 +/- 1.7, 17.4 +/- 1.7, respect ively). A smaller but significant increase was observed in laminae IV- VII and X. Under dark field illumination, an increase in the number of densely stained neurons in laminae I-III was also observed to peak at 6 h. A greater percentage of the neurons observed under bright field illumination were visible under dark field illumination at 6 h (47%) c ompared to control (18%), suggesting an increase in the enzymatic acti vity of neurons in laminae I-III in addition to the increase in the nu mber of neurons with threshold levels of NADPH-diaphorase activity. Th ere was no consistent increase in this ratio over time in laminae IV-V II or X. Six hours following carrageenan, there was a bilateral 50% in crease in the density of NADPH-diaphorase staining in the neuropil in the medial laminae I-III. Both spinal neurons and primary afferent axo ns contributed to this bilateral increase in staining as the number of NADPH-diaphorase stained dorsal root ganglion cell bodies increased 4 7% over control. In addition to the increase in staining in the lumbar spinal cord, at 6 h post carrageenan, there was a bilateral 23% incre ase over control in the number of stained neurons in the cervical dors al spinal cord. For comparative purposes, the distribution of Fos-immu noreactive nuclei was compared to NADPH-diaphorase 6 h post carrageena n. The distribution of Fos-immunoreactive nuclei differed from the NAD PH-diaphorase stained neurons, suggesting that two separate population s of neurons were stained. Unilateral hindpaw inflammation did not res ult in an increase in NADPH-diaphorase activity in the periaqueductal gray or the ventroposteriolateral thalamic nucleus. The relationship o f NADPH-diaphorase to nitric oxide is discussed and it is concluded th at NADPH-diaphorase, while labeling nitric oxide-producing neurons, is not an exclusive marker for nitric oxide synthase following unilatera l hindpaw inflammation. A role for NADPH-diaphorase in general excitat ory mechanisms in the spinal cord is proposed.