Specific innervation of the rat thalamus by grafted noradrenergic locus coeruleus neurons

Growth and distribution of noradrenaline (NA) fibres from the implant into the thalamus of host rats were examined at 5-13 months after the implantation by immunohistochemistry using NA or tyrosine hydroxylase antisera. Cell suspension dissociated from the locus coeruleus (LC) region of 14-day-old rat fetuses was implanted into the center of the unilateral thalamus in adult rats from which the noradrenergic afferents to the thalamus had been eliminated with 6-hydroxydopamine treatment. A dense network of varicose NA-immunoreactive (NA-LR) fibres extended laterally into the posterior thalamic nuclear group and the ventral posterolateral thalamic nucleus from the implant in a pattern similar to that the intrinsic noradrenergic fibres form in the normal thalamus, i.e. laterally rich and medially poor NA fibres. Electron microscopic observations revealed that varicosities of NA-LR fibres formed symmetrical as well as asymmetrical axodendritic synapses and axo-axonic synapses with the host neurons as seen in the normal thalamus. Labelled dendrite-like fibres of graft origin penetrated deep into the host brain and received afferents from non-labelled axon terminals. Varicosities of NA-IR fibres in the LC implanted animal formed axo-dendritic synapses at the higher ratio than those in the normal animal did. These results show that implanted fetal noradrenergic neurons innervate target regions of the thalamus specifically as the noradrenergic fibres in the normal thalamus do and maintain the innervation for a long time in the noradrenergically denervated rats.