Dual axonal terminations from the retrosplenial and visual association cortices in the laterodorsal thalamic nucleus of the rat.

Light and electron microscopic tracing studies were conducted to assess the synaptic organization in the laterodorsal thalamic nucleus (LD) of the rat and the laminar origins of corticothalamic terminals from the retrosplenial and visual association cortices to LD. A survey of the general ultrastructure of LD revealed at least three types of presynaptic terminals identified on the basis of size, synaptic vesicle morphology, and synaptic membrane specializations: (1) small axon terminals with round synaptic vesicles (SR), which accounted for the majority of terminal profiles and made asymmetric synaptic contacts predominantly with small dendritic shafts and spines; (2) large axon terminals with round synaptic vesicles (LR), which formed asymmetric synaptic contacts mainly with large dendritic shafts; and (3) small to medium-size axon terminals with pleomorphic synaptic vesicles (SMP), which symmetrically synapsed with a wide range of postsynaptic structures from cell bodies to small dendrites. Synaptic glomeruli were identified, whereas no presynaptic dendrites were found. To characterize and identify corticothalamic terminals arising from the retrosplenial and visual association cortices that project to LD, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into these cortices. Axons anterogradely labeled with WGA-HRP ended in both SR and LR terminals. On the other hand, dextran-tetramethylrhodamine injected into LD as a retrograde fluorescent tracer labeled large pyramidal cells of layer V as well as small round or multiform cells of layer VI in the retrosplenial and visual association cortices. These findings provide the possibility that corticothalamic terminations from cortical neurons in layer V end as LR terminals, while those from neurons in layer VI end as SR boutons.

Morphological features of cat cervicothalamic tract terminations in different target regions.

Using biotinylated dextran amine to label cervicothalamic tract terminations of cats, three types of terminal arrangements were recognized. The ventral posterior lateral nucleus contains the largest proportion of the cervicothalamic tract terminals (79%) and most (72%) of these are type I terminals (form compact clusters of 5-30 boutons). In contrast, type II (form less compact clusters of 3-10 boutons) and type III (widely spaced boutons along thin axons) terminals dominate in the medial nucleus of the posterior complex (78%) and in the ventral periphery of the ventrobasal complex (86%). In the magnocellular medial geniculate nucleus, type I terminals (38%) are found close to medially located clusters of Cat-301 immunoreactive neurons, whereas type II and type III terminals locate in the surrounding Cat-301-negative regions. These findings indicate a high degree of synaptic security in the transmission between cervicothalamic tract fibers and neurons in the ventral posterior lateral nucleus and highlight the role of this nucleus in faithful transmission of cervicothalamic tract input to the cerebral cortex. Also, the Cat-301-positive neurons in the magnocellular medial geniculate nucleus may faithfully transmit cervicothalamic tract signals. The domination of type II and type III terminals in the medial nucleus of the posterior complex and in the ventral periphery of the ventrobasal complex indicates a more divergent cervicothalamic input to these regions, in line with the large receptive fields and multimodal responses of neurons in the posterior complex.