Afferents from the colliculus, cortex, and retina have distinct terminal morphologies in the lateral posterior thalamic nucleus

Abstract

We have examined the morphology of afferent endings that originate in three distinct cell groups and terminate in the lateral posterior nucleus of the thalamus (LP). Retino-LP projections were sparse, occurred throughout the nucleus, and could be classified into 1) simple en passant varicosities and terminal swellings found on poorly branched fibers in all LP subdivisions, 2) string-like configurations of varicosities detected largely in the medial subdivision of the LP, and 3) terminals resembling retinogeniculate endings occurring mainly in the rostral part of the superficial subdivision of the LP adjacent to the dorsal nucleus of the lateral geniculate body. Cortico-LP terminals fell into three classes: 1) single varicosities decorating the tips of short appendages on fine preterminal and terminal axons; 2) tiny, round varicosities studding the axon shaft; and 3) boutons of variable shape visible on medium-caliber corticothalamic fibers. Tecto-LP terminals exhibited a large variation in morphology and density. Those found most commonly could be classified into two groups: 1) individual swellings and 2) terminal clusters arranged in a tubular configuration that enclosed a central channel, most likely occupied by the dendrite of a postsynaptic neuron. An unusual tecto-LP terminal consisted of an ovoid swelling (up to 20 microns in the long axis) from which emerged several long, thin extensions and was seen at the tips of large-diameter axons. These results show that, despite having overlapping projection zones, each set of afferents that projects to the LP elaborates terminal specializations that are structurally distinct from others projecting to the same target area.

Synaptic organization of the tectorecipient zone of the rat lateral posterior nucleus

“…Robson and Hall (1977) first described tectothalamic terminals in the grey squirrel pulvinar nucleus as RM profiles, and illustrated the manner in which they encircle relatively large caliber dendrites. Similar patterns have also been observed in the squirrel monkey (Mathers, 1971), Rhesus monkey (Partlow et al, 1977), hamster (Crain and Hall, 1980; Ling et al, 1997), cat (Kelly et al, 2003), and tree shrew (Chomsung et al, 2008). …”

“…However, a previous study in the hamster concluded that there is a large degree of convergence in the rodent tecto-LPN pathway (Mooney et al, 1984). Thus the available results suggests that individual LPN neurons can be innervated by multiple tectal axons, and each tectal axon can make multiple synaptic connections with LPN cells via dense terminal clusters (Ling et al, 1997; current study Figure 7C). …”

“…Recent studies of the primate mediodorsal nucleus indicate that inputs from the SC can significantly impact thalamic and cortical activity patterns (Wurtz and Sommer, 2004) yet, where examined, tectothalamic terminals form unique synaptic arrangements that do not fall into driver or modulator categories (Mathers, 1971; Partlow et al, 1977; Crain and Hall, 1980; Ling et al, 1997; Kelly et al, 2003; Chomsung et al, 2008). Furthermore, we previously found that the membrane properties and responses to stimulation of cortical fibers recorded within the caudal LPN of the rat are distinct from those recorded in the rostral LPN (Li et al, 2003b; Li et al, 2003c).…”

Resolving the Detailed Structure of Cortical and Thalamic Neurons in the Adult Rat Brain with Refined Biotinylated Dextran Amine Labeling

“…7E). A third type of corticothalamic axon, which has been documented previously in the hamster [67], but not in the rat, is revealed by BDA labeling to have large varicosities from which medium caliber collaterals and boutons originate (Fig. 7D).…”

Frequency-Dependent Release of Substance P Mediates Heterosynaptic Potentiation of Glutamatergic Synaptic Responses in the Rat Visual Thalamus

“…Smith et al (2007) further proposed that it may be the collective activities of multiple convergent integrator inputs that are critical for the formation of the postsynaptic neuron's receptive field. This concept fits well with the known properties of the LPN; receptive fields within the tectorecipient LPN are much larger than those recorded in the superficial layers of the SC where tecto-LPN cells are located (Abramson and Chalupa 1988;Casanova and Molotchnikoff 1990;Chalupa et al 1983;Hilbig et al 2000;Hutsler and Chalupa 1991;Ling et al 1997;Major et al 2000). In the hamster, Mooney et al (1984) demonstrated that receptive fields of LPN cells are on average 10-fold larger than those of tecto-LPN cells.…”