Synaptic development of the mouse dorsal lateral geniculate nucleus.

The dorsal lateral geniculate nucleus (dLGN) of the thalamus has served as an important model for understanding many aspects of visual system development including early pathfinding, synaptogenesis, visual map formation, and the activity-dependent refinement of synaptic connections. In recent years the mouse has become a major experimental platform for detailing key elements involved in these events. Despite the abundance of studies done in the mouse, little is known about the underlying synaptic organization of the developing dLGN. To date, studies have focused largely on the development of retinogeniculate connections (Guido, 2008). Initially, dLGN cells are binocularly innervated and receive as many as a dozen or so separate retinal inputs. By natural eye opening (P12–P14) many of these inputs have been eliminated so that dLGN cells receive monocular innervation from just one or a few retinal ganglion cells. During this pruning phase, the remaining retinogeniculate synapses experience a several fold increase in strength (Chen and Regehr, 2000). Little is known regarding the ultrastructural changes underlying these events. In adult tissue, retinal terminals in the dLGN have been well studied and are quite distinct (Szentagothai et al., 1966; Guillery, 1969; Guillery, 1971; Robson and Mason, 1979; Hamos et al., 1987). They are known as RLP profiles because of their round shaped vesicles, large size, and pale mitochondria. Indeed, retinal terminals are the largest terminals in the dLGN, and each bouton forms multiple synaptic contacts (Rafols and Valverde, 1973; Rapisardi and Miles, 1984; Wilson et al., 1984). Whether these features are apparent at early ages, and how the remodeling process takes shape at the ultrastructural level, remain unexplored. Adult retinogeniculate terminals also contact dendritic terminals (F2 profiles), which have been identified as originating from interneurons (Famiglietti, 1970; Famiglietti and Peters, 1972; Rafols and Valverde, 1973; Lieberman and Webster, 1974; Hamos et al., 1985) that utilize γ-aminobutyric acid (GABA) as a neurotransmitter (Montero and Singer, 1985; Montero, 1991; Fitzpatrick et al., 1984). Unique “triadic” arrangements are formed when a single retinal terminal is pre-synaptic to both a relay cell dendrite and an F2 profile, which are themselves connected by a synapse. These arrangements often occur within a glomerulus, a complex zone of multiple synapses encased by a glial sheath (Sherman, 2004). In other species, approximately 20% of the retinogeniculate synapses are formed with F2 profiles and the remainder with relay cell dendrites (Wilson and Hendrickson, 1981; Erisir et al., 1998; Van Horn et al., 2000; Datskovskaia et al., 2001; Li et al., 2003). However, although triadic arrangements have been identified in the adult mouse dLGN (Rafols and Valverde, 1973), it is not clear how prevalent these synapses are, because mice have far fewer intrinsic interneurons than higher mammals (Arcelli et al., 1997). It is also not clear when they first appear during development because the maturation of some inhibitory synapses in the rodent thalamus is delayed until postnatal ages (DeBiasi et al., 1997; Ziburkus et al., 2003). Finally, although retinal inputs provide the primary excitatory drive for dLGN cells, most of the synaptic connections in the dLGN arise from nonretinal sources (Sherman and Guillery, 2002). In the adult dLGN, the cortex and brainstem provide glutamatergic and cholinergic inputs, respectively (Erisir et al., 1997a,b), in the form of small profiles with densely packed round vesicles and dark mitochondria (RSD profiles). Additional GABAergic inputs arise from the thalamic reticular nucleus (TRN), pretectum, and interneuron axons in the form of medium-sized terminals that contain densely packed vesicles (F1 profiles), distinguishing them from the more loosely packed vesicles contained within F2 profiles (Ohara et al., 1980; Montero and Scott, 1981; Fitzpatrick et al., 1984; Wilson et al., 1984; Hamos et al., 1985; Montero and Singer, 1985; Cucchiaro et al., 1991a,b; Arcelli et al., 1997; Wang et al., 2001; Wang et al., 2002; Li et al., 2003). However, little if anything is known about how, where, and when these connections emerge or coordinate with the refinement of retinogeniculate connections. To address these questions, we explored the synaptic organization of the developing mouse dLGN in the common pigmented C57/BL6 strain, first by recording the synaptic responses evoked by electrical stimulation of optic tract axons, and second by examining the ultrastructure of identified synapses. To aid in the characterization of inhibitory elements, we also made use of postembedding immunocytochemical staining for GABA.