Descending supraspinal pathways in amphibians. II. Distribution and originof the catecholaminergic innervation of the spinal cord

Immunohistochemical studies with antibodies against tyrosine hydroxylase, d opamine, and noradrenaline have revealed that the spinal cord of anuran, ur odele, and gymnophionan (apodan) amphibians is abundantly innervated by cat echolaminergic (CA) fibers and terminals. Because intraspinal cells occur i n all three orders of amphibians CA, it is unclear to what extent the CA in nervation of the spinal cord is of supraspinal origin. In a previous study, we showed that many cell groups throughout the forebrain and brainstem pro ject to the spinal cord of two anurans (the green frog, Rana perezi, and th e clawed toad, Xenopus laevis), a urodele (the Iberian ribbed newt, Pleurod eles waltl), and a gymnophionan (the Mexican caecilian, Dermophis mexicanus ). To determine the exact site of origin of the supraspinal CA innervation of the amphibian spinal cord, retrograde tracing techniques were combined w ith immunohistochemistry for tyrosine hydroxylase in the same sections. The double-labeling experiments demonstrated that four brain centers provide C A innervation to the amphibian spinal cord: 1.) the ventrolateral component of the posterior tubercle in the mammillary region, 2.) the periventricula r nucleus of the zona incerta in the ventral thalamus, 3.) the locus coerul eus, and 4.) the nucleus of the solitary tract. This pattern holds for all three orders of amphibians, except for the CA projection from the nucleus o f the solitary tract in gymnophionans. There are differences in the strengt h of the projections (based on the number of double-labeled cells), but in general, spinal functions in amphibians are controled by CA innervation fro m brain centers that can easily be compared with their counterparts in amni otes. The organization of the CA input to the spinal cord of amphibians is largely similar to that described for mammals. Nevertheless, by using a seg mental approach of the CNS, a remarkable difference was observed with respe ct to the diencephalic CA projections. (C) 2001 Wiley-Liss, Inc.