Vibrissae-evoked activity in the somatosensory thalamus of rat pups: an intracellular study

Spontaneous and sensory-evoked neuronal activity plays a decisive role in network formation during postnatal development. The thalamus is a major gateway for sensory outputs to the cortex, so that thalamic neuronal activity in newborn animals might be crucial for maturation of the thalamocortical network. The sensory-evoked intracellular thalamic activity and signal propagation in newborn animals remain largely unknown. Here we performed local field potential (LFP), juxtacellular, and patch clamp recordings in the somatosensory thalamus of urethane anesthetized rats at postnatal days 6-7 (P6-7, both sexes) with one whisker stimulation. To reach the thalamus with the electrodes the majority of the overlying cortex and hippocampus were removed. Deflection of only one (the principal) whisker induced spikes in a particular thalamic cell. Sensory stimulation evoked excitatory and inhibitory postsynaptic events in thalamocortical cells. Up to 5-10 sensory-evoked large-amplitude excitatory events followed with 100-200 ms inter-event intervals, while multiple inhibitory events tended to form 20-40 ms inter-event intervals. Large-amplitude excitatory events produced spike bursts with an intraburst frequency of 50-100 Hz and/or short plateau potentials in thalamocortical cells. Inhibitory events could down-modulate evoked spiking or prevented a depolarization block. Juxtacellular recordings confirmed the partial inactivation of spikes during short plateau potentials. Excitatory events evoked low-threshold spikes (LTS) in thalamocortical cells, but, in agreement with previously reported results, hyperpolarizing current pulses generated weak LTS without spike bursts. We conclude that thalamic neuronal activity in rat pups is determined by relatively weak and slow intrinsic membrane currents and relatively strong synapses that might underlay immature forms of thalamocortical synchrony and signal propagation.