We describe a means of visualizing the same neuron in the superior cervical ganglion of young adult mice over intervals of up to 3 months. The dendrites of these neurons change during this interval; some branches retract, others elongate, and still others appear to form de novo. Thus, neuronal dendrites in this part of the nervous system are subject to continual change beyond what is usually considered the developmental period. The remodeling of postsynaptic processes further implies that the synaptic connections made onto these cells undergo substantial rearrangement well into adulthood.
Dendritic arborizations of neurons in the adult rat superior cervical ganglion were measured in control ganglia and in ganglia innervating peripheral targets that were relatively larger or smaller than normal. The relative size of the target--the submandibular gland in these experiments--was manipulated during development by changing the ratio between the amount of target tissue and the number of innervating ganglion cells. Thus, ligating the submandibular salivary duct reduced the size of the gland, whereas partially denervating the gland produced a relatively larger target by making a smaller number of ganglion cells innervate a gland of normal size. Neurons innervating targets that were smaller than normal had significantly smaller dendritic arborizations and cell bodies than control cells. Conversely, neurons projecting to relatively larger than normal targets had larger dendritic arborizations and cell bodies, and more primary dendritic branches. Such cells were also innervated by a larger than normal number of preganglionic inputs. A similar change in dendritic geometry was observed when relative target size was increased after cutting the cervical sympathetic trunk, showing that target regulation of dendritic geometry is not dependent on ganglion cell activity or the presence of presynaptic innervation. Dendrites in the superior cervical ganglion normally grow in parallel with body size throughout life (Purves et al., 1986a; Voyvodic, 1987a). The present results imply that an important aspect of dendritic growth is an ongoing responsiveness of ganglion cells to feedback signals arising from the peripheral targets they innervate.
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