Local Connections of Excitatory Neurons to Corticothalamic Neurons in the Rat Barrel Cortex

Abstract

Corticothalamic projection neurons in the cerebral cortex constitute an important component of the thalamocortical reciprocal circuit, an essential input/output organization for cortical information processing. However, the spatial organization of local excitatory connections to corticothalamic neurons is only partially understood. In the present study, we first developed an adenovirus vector expressing somatodendritic membrane-targeted green fluorescent protein. After injection of the adenovirus vector into the ventrobasal thalamic complex, a band of layer (L) 6 corticothalamic neurons in the rat barrel cortex were retrogradely labeled. In addition to their cell bodies, fine dendritic spines of corticothalamic neurons were well visualized without the labeling of their axon collaterals or thalamocortical axons. In cortical slices containing retrogradely labeled L6 corticothalamic neurons, we intracellularly stained single pyramidal/spiny neurons of L2-6. We examined the spatial distribution of contact sites between the local axon collaterals of each pyramidal neuron and the dendrites of corticothalamic neurons. We found that corticothalamic neurons received strong and focused connections from L4 neurons just above them, and that the most numerous nearby and distant sources of local excitatory connections to corticothalamic neurons were corticothalamic neurons themselves and L6 putative corticocortical neurons, respectively. These results suggest that L4 neurons may serve as an important source of local excitatory inputs in shaping the cortical modulation of thalamic activity.

Local connections of excitatory neurons in motor-associated cortical areas of the rat

“…The appositions formed between the local axon collaterals of the intracellularly labeled pyramidal neurons and the dendrites of CSNs were traced as shown in Figure 5 . In a different set of experiments, about 60–77% of appositions were electron-microscopically confirmed to make axodendritic synaptic contacts of asymmetric type mainly on dendritic spines ( Figures 4I–K′ and 8O–R ; Cho et al, 2004b; Tanaka et al, 2011b), suggesting that the number of appositions could be applied as a quantitative indicator of synaptic connections.…”

“…The method was basically composed of the specific retrograde or transgenic labeling approach and conventional intracellular staining technique: on one hand, the information-receiving sites (cell body and dendrites) of a functional group of cortical neurons were visualized by the Golgi stain-like retrograde labeling technique (Kaneko et al, 1996, 2000; Cho et al, 2004b; Tanaka et al, 2011a) or by the transgenic method for the expression of somatodendritic membrane-targeted green fluorescent protein (GFP; Tanaka et al, 2011b; Kameda et al, 2012); and, on the other hand, the local axonal arborization of single neurons was labeled by the sharp electrode intracellular (Kaneko et al, 2000; Cho et al, 2004b; Tanaka et al, 2011b) or whole-cell clamp recording technique (Tanaka et al, 2011a) with thick cortical slices. Subsequently, the local connection of single cortical neurons to the functional neuron group was investigated morphologically and quantitatively.…”

Axon topography of layer 6 spiny cells to orientation map in the primary visual cortex of the cat (area 18)

“…From the circuitry point of view, L6 cells are implicated at least in two functional streams: the major intracortical input sources to L4 on the one hand and as one of the output sources to subcortical regions, mainly LGN and claustrum, on the other hand. Electron microscopy of L6 cells estimated that they account for 20–40 % of all synapses on L4 spiny stellate cells (Ahmed et al 1994, 1997; Binzegger et al 2004; Tanaka et al 2011; Pichon et al 2012). In addition to this, L6 outputs were shown to activate inhibitory neurons too (West et al 2006; Olsen et al 2012; Bortone et al 2014), although the relative proportion of the targeted excitatory and inhibitory cells is still a matter of debate (McGuire et al 1984; Somogyi 1989; Ahmed et al 1994, 1997; Staiger et al 1996).…”

Synaptic Microcircuits in the Barrel Cortex

“…L4 spiny neurons innervate also L6 pyramidal cells, at least those in sublamina 6A albeit at a very low connectivity ratio of ~ 3 % [24,49,54,55]. However, the innervation domain of L4 spiny stellate cells and L4 star pyramids with L6A pyramidal cells is distinct: the former establish synaptic contacts with apical tuft dendrites of L6 pyramidal cells (which terminate in layer 4) while the latter target predominately basal dendrites [54].…”

“…Their axons have been described to be extremely short, as found both in vitro and in vivo studies. This type of CT L6A pyramidal cells has been proposed to receive a strong and focussed synaptic input from L4 spiny neurons in their home 'barrel column', indicating that neurons in this layer are involved in shaping the cortical modulation of activity in the somatosensory thalamus [54,55].…”