Distinct firing properties of higher order thalamic relay neurons

It has been proposed that the thalamus is composed of at least two types of nuclei. First-order relay nuclei transmit signals from the periphery to the cortex while higher order nuclei may route information from one cortical area to another. Although much is known about the functional properties of relay neurons in first-order nuclei, little is known about relay neurons belonging to higher-order nuclei. We investigated the electrophysiological properties of relay cells in a higher-order thalamic nucleus using in vitro intracellular recordings from thalamic slices of the rat's lateral posterior nucleus (LPN). We found neurons of the LPN possess many of the same membrane properties as first-order relay neurons. These included low-threshold calcium spikes (IT) and burst firing, a mixed cation conductance (IH) that prevented membrane hyperpolarization, and a transient K+ conductance that delayed spike firing (IA). The repetitive firing characteristics of LPN neurons were more distinct. One group of cells, located in the more caudal regions of the LPN responded to depolarizing current pulses with a train of action potentials or in a regular spiking (RS) mode. This form of firing showed a steep but highly linear increase in firing frequency with increasing levels of membrane depolarization. Another group of cells, located in the more rostral regions of the LPN, responded to depolarizing current pulses with clusters of high-frequency bursts or in a clustered spiking (CS) mode. The overall firing frequency rose nonlinearly with membrane depolarization, but the frequency of a given burst remained relatively constant. The caudal LPN receives input from the superior colliculus, whereas the rostral LPN receives input from layers V and VI of the visual cortex. Thus the RS and CS cells may be driven by subcortical and cortical inputs respectively, and the distinct temporal properties of their response modes may be a necessary component of the LPN circuitry.