Plasticity in the nucleus gracilis of the rat

Abstract

The nucleus gracilis of the rat contains a somatotopic map of the hindquarter similar to that in other mammals. Acute deafferentation of this dorsal column nucleus (DCN) either by dorsal root transection (roots L4, 5, and 6), by transection of the spinal cord at L3, or by peripheral nerve transection (the sciatic and saphenous nerves) creates as expected a clearly demarcated region in which cells have no receptive fields (RFs). Plasticity in the nucleus gracilis after four types of chronic deafferentiation was investigated: Experiment 1—15 to 20 days after transection of L4, 5, and 6 dorsal roots, cells in the acutely unresponsive region were found to be excited from nearby intact afferent fibers. This expansion of the effective input of intact afferent fibers is known to occur in the spinal cord. Experiment 2—Chronic transection of the sciatic and saphenous nerves was not followed by expansion of the intact input although this does occur in the spinal cord. Experiment 3—Neonatal treatment with capsaicin which destroys unmyelinated afferent fibers produced adult animals with somatotopically organized nucleus gracilis but with cells which exhibited very large RFs. This enlargement is also known to occur in the spinal cord. Experiment 4—Local application of capsaicin to a sciatic nerve of an adult enlarged the RFs of spinal cord cells after some days but had no effect on the functional organization of nucleus gracilis. Expansion of the RF of nucleus gracilis cells was, therefore, seen after dorsal root transection and after neonatal application of capsaicin but not after peripheral nerve transection or application of capsaicin to the adult single nerve. All four maneuvers produced RF expansion in the spinal cord. We conclude that the DCN has a more limited ability to respond to deafferentation than the spinal cord, and suggested that the former lacks a mechanism of connectivity control. The results are consistent with the hypothesis that primary afferent C fibers which innervate the spinal cord but not the DCN, play a role in the formation and stability of RFs formed by A fibers in adult animals.