Differences between embryos and adults in the plasticity of somatosensory afferents to the axolotl tectum.

The plasticity of somatosensory afferents in the adult axolotl tectum was studied in two ways. First, normal adult axolotls were monocularly enucleated, and second, adult animals which had been monocular since embryogenesis had the tectum contralateral to the remaining eye excised. After a survival time of about one year the brains of these animals were studied electrophysiologically and histochemically. In the enucleated adults, the deprived tectum lacked visual activity, and the acetylcholinesterase staining, dense in the superficial neuropil of the normally innervated tectum, was much reduced. Somatosensory units remained deep in the tectal neuropil, as in normally innervated tecta. Serotonergic terminals associated with somatosensory responses were correspondingly largely restricted to the deeper tectal neuropil in these animals. This result is in contrast to that of embryonic enucleation where somatosensory activity and serotonergic terminals are found in the superficial tectal neuropil of the adult. Autoradiographic results from the tectal excision experiments showed that retinal axons in adults regrew to innervate the ipsilateral tectum when deprived of their normal targets. Associated with the ipsilateral innervation both visual units and acetylcholinesterase staining were found in the superficial tectum. In spite of this change in the visual innervation of the tectum, there was little difference in the distribution of somatosensory responses or serotonergic terminals; both remained relatively superficial and appeared to be unaffected by the late ingrowing retinal axons. This result is in contrast to the findings from embryonic transplants to genetically eyeless embryos which, as adults, show somatosensory responses and serotonergic endings restricted to the deeper tectal neuropil.