Sherman lab gains insight into thalamocortical circuitry

In recent years, the laboratory of Professor S. Murray Sherman has been using cutting edge research techniques to examine and understand the functional role of glutamatergic pathways in the brain. In the past, the laboratory has produced an extensive amount of evidence for the existence of two different types of glutamatergic input to thalamus: driver inputs, which are information-bearing, and modulatory inputs which are not information-bearing but rather, like their name implies, function as modulators of information transmission. Whereas this distinction was initially thought to exist only for inputs into thalamus originating in the periphery or cortex, recent evidence from the lab suggests that it may also apply to other parts of the brain. More specifically, it has been shown that the same distinction applies to inputs from one cortical area to another. Covic and Sherman (2011) showed that nature of inputs (driver vs modulatory) between the primary and secondary auditory cortices depends on the layer of origin and the layer of termination. Similar findings were also reported by De Pasquale and Sherman (2011) for inputs between the primary and secondary visual cortices. 

A perhaps somewhat more surprising finding however was that the driver/modulator distinction may also apply for thalamic inputs to cortex. Traditionally, the thalamus was seen as a simple relay of information and was thus assumed to convey driving input from the periphery to cortex. In a series of three related papers Viaene, Petrof and Sherman showed that thalamic input to somatosensory and auditory cortices is not homogenous in nature. Whereas thalamic input to layer 4 of a primary sensory cortex is always of driver nature, thalamic input to supragranular and subgranular layers is more diverse. In the case of the supragranular layers, thalamic input appears to be predominantly (but not exclusively) modulatory (Viaene et al, 2011a), while several different sub-classes of driving-like input were described for the subgranular layers (Viaene et al, 2011b). In addition, input from the posterior medial nucleus, a higher order thalamic nucleus, to the primary somatosensory cortex was shown to be of modulatory nature, but input from the same nucleus to layer 4 of the secondary somatosensory cortex was shown to posses the characteristics of a driver input (Viaene et al, 2011c). These findings suggest that the role of the thalamus is more diverse than initially thought and that thalamus does not simply convey information to cortex but it can also directly modulate cortical activity.

In recent years another exciting point of focus for the Sherman lab has been the examination of the parallel cortico-cortical and trans-thalamic pathways. Theyel, Llano and Sherman, (2010) showed that, in addition to direct cortico-cortical communication, cortical areas can interact via the thalamus. Through descending projections, a primary sensory cortical area can drive a thalamic nucleus (e.g. the posterior medial nucleus), which in turn can drive a secondary sensory cortical area.  In a recent review paper by Sherman and Guillery (2011), the authors summarize and discuss the differences between the two pathways and the functional importance for the existence of two parallel pathways of sensory processing. Related to this topic, Petrof, Viaene and Sherman (2012) report that in the subgranular layers of the primary somatosensory, auditory and visual cortices there is virtually no overlap between the two populations of cells that project to thalamus and those that project to a secondary sensory cortical area, meaning amongst others, that the cortico-cortical and transthalamic pathways emanate from different cellular populations within a primary cortical area.

Related References

Covic EN, Sherman SM (2011) Synaptic Properties of Connections between the Primary and Secondary Auditory Cortices in Mice. Cereb Cortex. 21(11):2425-41

De Pasquale R, Sherman, SM (2011) Synaptic Properties of Corticocortical Connections between the Primary and Secondary Visual Cortical Areas in the Mouse. J Neurosc, In Press

Petrof I, Viaene AN, Sherman SM (2012). Two populations of corticothalamic and interareal corticocortical cells in the subgranular layers of the mouse primary sensory cortices. J Comp Neurol, In Press

Sherman SM, Guillery RW (2011) Distinct functions for direct and transthalamic corticocortical connections. J Neurophys. 106:1068-1077

Theyel B, Llano D, and S. M. Sherman (2010) The Corticothalamocortical Circuit Drives Higher-Order Cortex in the Mouse.  Nat. Neuroscience. Jan 13:84-88.

Viaene AN, Petrof I, Sherman SM. (2011) Synaptic properties of thalamic input to layers 2/3 and 4 of primary somatosensory and auditory cortices. J Neurophys. Jan;105(1):279-92.

Viaene AN, Petrof I, Sherman SM. (2011) Synaptic properties of thalamic input to the subgranular layers of primary somatosensory and auditory cortices in the mouse. J Neurosc. Sep 7;31(36):12738-47

Viaene AN, Petrof I, Sherman SM.  (2011) Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse. PNAS, 108(44):18156-18161