Processing of Periodic Whisker Deflections By Neurons in the Ventroposterior Medial and Thalamic Reticular Nuclei

Abstract

Rats employ rhythmic whisker movements to sample information in their sensory environment. To study frequency tuning and filtering characteristics of thalamic circuitry, we recorded single-unit responses of ventroposterior medial (VPm) and thalamic reticular (Rt) neurons to 1- to 40-Hz sinusoidal and pulsatile whisker deflection in lightly narcotized rats. Neuronal entrainment was assessed by a measure of the relative modulation (RM) of firing at the stimulus frequency given by the first harmonic (F1) of the cycle time histogram divided by the mean firing rate (F0). VPm signaling of both sinusoidal and periodic pulsatile whisker movements improved gradually over 1-16 and was maximal at 20-40 Hz. By contrast, the RM of Rt responses increased over 1-8 Hz, but deteriorated progressively over the 12- to 40-Hz range. In Rt, response adaptation occurred at lower stimulus frequencies and to a greater extent than in VPm. Within a train of high-frequency stimuli, Rt responses progressively decremented, possibly due to the accumulation of inhibition, whereas those of VPm neurons augmented. Mean firing rates in Rt increased 42 spikes/s over 1-40 Hz, providing tonic (low RM) inhibition during high-frequency stimulation that may enhance VPm signal-to-noise ratios. Consistent with this view, VPm mean firing rates increased only 13 spikes/s over 1-40 Hz, and inter-deflection activity was suppressed to a greater extent than stimulus-evoked responses. Rt inhibition is likely to act in concert with actions of neuromodulators in optimizing thalamic temporal signaling of high-frequency whisker movements.

Early Development of the Thalamic Inhibitory Feedback Loop in the Primary Somatosensory System of the Newborn Mice

“…Our results show also that the efficacy of synaptic transmission in the thalamic nVPM-nRT feedback loop is strong at the end of the first postnatal week, suggesting that it may already ensure a relatively good temporal coding of the sensory information (Hartings et al, 2003). The GABA A -mediated IPSC will undergo more changes, such as a reduction of their duration, later in development Huntsman and Huguenard, 2000;Peden et al, 2008), suggesting that the thalamic inhibitory feedback loop has not yet reached its adult stage at the end of the first postnatal week.…”

Thalamocortical transformations of periodic stimuli: the effect of stimulus velocity and synaptic short-term depression in the vibrissa–barrel system

“…4). Both pulses and sinusoidal stimulation evoke thalamic responses which resemble a Gamma function (Sosnik et al 2001;Hartings et al 2003;Khatri et al 2004). Moreover, thalamic CPSTH's in response to high f sinusoidal stimulation show a secondary smaller peak which we neglected [probably produced by inhibition coming from reticular cells (Golomb et al 2006)].…”

“…We consider the effect of thalamic adaptation, defined as the decrease of the peak thalamic response, C, with stimulation frequency (Ahissar et al 2000;Hartings et al 2003;Castro-Alamancos 2002), which we model with the following dependence:…”

“…Recordings of VPm cells applying different types of pulse stimulation found that the CPSTH shows little adaptation with f in the range of moderate frequencies (e.g. ∼15-20% at f ∼ 30 Hz) (Hartings and Simons 1998;Hartings et al 2003;Sosnik et al 2001). Another important study reveals that during quiescent states, thalamic responses to repetitive whisker stimulation are strongly suppressed at frequencies above f ∼ 2 Hz.…”

“…The available data is limited to the interval f = 0-40 Hz where it increases by about 20% (Khatri et al 2004;Hartings et al 2003). Based on this, we take the extreme assumption that the ThSR remains constant in the whole interval f = 0-200 Hz.…”