Mapping by Laser Photostimulation of Connections Between the Thalamic Reticular and Ventral Posterior Lateral Nuclei in the Rat

Abstract

We used laser scanning photostimulation through a focused UV laser of caged glutamate in an in vitro slice preparation through the rat's somatosensory thalamus to study topography and connectivity between the thalamic reticular nucleus and ventral posterior lateral nucleus. This enabled us to focally stimulate the soma or dendrites of reticular neurons. We were thus able to confirm and extend previous observations based mainly on neuroanatomical pathway tracing techniques: the projections from the thalamic reticular nucleus to the ventral posterior lateral nucleus have precise topography. The reticular zone, which we refer to as a "footprint," within which photostimulation evoked inhibitory postsynaptic currents (IPSCs) in relay cells, was relatively small and oval, with the long axis being parallel to the border between the thalamic reticular nucleus and ventral posterior lateral nucleus. These evoked IPSCs were large, and by using appropriate GABA antagonists, we were able to show both GABA(A) and GABA(B) components. This suggests that photostimulation strongly activated reticular neurons. Finally, we were able to activate a disynaptic relay cell-to-reticular-to- relay cell pathway by evoking IPSCs in relay cells from photostimulation of the region surrounding a recorded relay cell. This, too, suggests strong responses of relay cells, responses strong enough to evoke spiking in their postsynaptic reticular targets. The regions of photostimulation for these disynaptic responses were much larger than the above-mentioned reticular footprints, and this suggests that reticulothalamic axon arbors are less widespread than thalamoreticular arbors, that there is more convergence in thalamoreticular connections than in reticulothalamic connections, or both.

Distinct Electrical and Chemical Connectivity Maps in the Thalamic Reticular Nucleus: Potential Roles in Synchronization and Sensation

“…In addition, nRt cell axons give rise to sparse intranuclear collaterals generally not extending beyond the dendritic arborization of the cell (Scheibel and Scheibel, 1966;Yen et al, 1985;De Biasi et al, 1988;Uhlrich et al, 1991;Cox et al, 1996;Sanchez-Vives et al, 1997), suggesting the existence of chemical axodendritic and/or axosomatic synapses between reticular cells. Physiological investigations confirmed functional inhibitory GABAergic input onto nRt cells (Ahlsen and Lindstrom, 1982;Ulrich and Huguenard, 1995), at least some of which arises from within nRt (Sanchez-Vives et al, 1997;Shu and McCormick, 2002;Zhang and Jones, 2004;Lam and Sherman, 2005). Dendrodendritic contacts have also been proposed to mediate inhibitory synaptic interactions (Deschenes et al, 1985;Yen et al, 1985;De Biasi et al, 1986;Williamson et al, 1994;Pinault et al, 1997).…”

“…Our mapping results do not provide direct information regarding the anatomical basis (axodendritic vs dendrodendritic) of intranuclear inhibition. Topographic and sensory modality-specific organization exist in nRt both longitudinally (sectors), as well as within the mediolateral extent of the nucleus (tiers) (Jones, 1975;Shosaku et al, 1984;Crabtree, 1996;Crabtree et al, 1998;Lam and Sherman, 2005). For example, neurons located in the central tier of somatosensory nRt project to the ventral posterior nuclei, whereas medial tier neurons connect to the posterior complex (Crabtree, 1996).…”

Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse

“…Nitroindolinyl-caged glutamate (Sigma-Aldrich) was added to the recirculating artificial cerebrospinal fluid (0.4 mM) for the experiments requiring glutamate photo-uncaging. A UV laser beam (DPSS Laser) was used to locally photolyse the caged compound over an 8 × 8 grid in a pseudorandom order (40)(41)(42)(43). The laser beam had an intensity of 20-80 mW, and the laser illumination lasted 2 ms (355 nm wavelength, frequencytripled Nd:YVO4, 100-kHz pulse repetition rate).…”

Tonotopic Control of Auditory Thalamus Frequency Tuning by Reticular Thalamic Neurons

“…There is also anatomical evidence for a coarse organization of the thalamo-reticular and reticulo-thalamic projections because within a given sensory system, the different thalamic nuclei project to and receive from different TRN subsectors (Coleman and Mitrofanis 1996;Crabtree 1996Crabtree , 1998Pinault et al 1995a,b). In a recent in vitro electrophysiological study, laser photostimulation of reticular neurons allowed a precise determination of the TRN area (the " footprint ") providing inhibitory inputs onto a given thalamic cell (Lam and Sherman 2005). Even if the size of the reticular footprint varied with the laser power, on average it was quite small (310 ϫ 117 m), and there was a spatial relation between the position of the footprint and the position of the thalamic cell, suggesting a topographic projection from the TRN to the VP nucleus.…”